Article [13] ST1300 - Brake Maintenance - Photos of dismantled SMC

I'm intrigued by the arrangement of the SMC etc in the braking circuit. It really is a mind-bending piece of engineering that I struggle to get my head around. Works well though.

The little one-way valve in the plastic cartridge allows flow in, but not easily out, and I'm guessing that may be to facilitate bleeding. Given that cartridge also has a tiny, always-open port, no pressure can be caught in the SMC provided this tiny port and the compensating port in the SMC remain open. When pedal pressure drops but the SMC remains engaged by front brake use, the tiny port is needed to allow fluid from the low-pressure side of the SMC to return to the pedal line when the SMC relaxes.

If I'm not mistaken the rear outer pistons which are powered by the movement of the SMC are at all times also exposed to at least the same hydraulic circuit pressure as the front centre pistons, as they are connected via the tiny compensating ports whenever the SMC is relaxed. So whether the front brake lever operates or not, all three rear pistons will be in action whenever the brake pedal is operated. Is that correct?

As far as I can see, the only reason for the SMC then is to operate the rear brake if the pedal is not pressed.
 
Hi all

Sorry to jump in on a post that hasn't been added to for a while but I have an issue with my SMC that looks like it may have been addressed by members here.

I am rebuilding my SMC and I have taken out and cleaned the little plastic valve in the SMC that is behind the black brake pipe port. Not thinking it mattered I didn't take note of the orientation of the valve when I removed it.

Does anyone know the correct orientation of the vale at all?

Many thanks in advance
 
The Inlet Port Cartridge.

This component really surprised me. I didn't expect to find that in there. I don't know what I expected to find under the cover - perhaps a spring and a ball bearing, but it wasn't this.

Anyway - the four components - A white circular gauze filter; A white plastic cylindrical body which contains a large hole and a tine hole; a blue cap which holds one end of the spring, while allowing fluid through a fine gauze; A tiny rubber seal to allow fluid into the inlet port but to prevent backflow. The gauze is to prevent any minute bits of debris from getting into the tiny pressure relief hole

I've knocked up a diagram to represent how they fit together.

View attachment 180935 View attachment 180936
Click to expand...
Hi, I have a VFR with linked brakes and SMC very similar in design. I had rear brake lock up and concluded the SMC at fault. I've taken it and stripped it yet to reassemble to test... I have two SMC in fact one from ebay. The port cartridge on both units does not appear to have ANY relief port hole, only a one way spring valve with ball bearing at the end... When testing both units off the bike it seems to not allow for fluid to flow back through the inlet port to the MC at the foot brake. Can you confirm? I had to bleed the caliper to return the pistons, and now cleaning the SMC (may need to rebuild or buy new- it's 22 years old!!). When playing with the SMC in the sink the relief port allows fluid back through it (when the cartridge is out). I've been on the VFR forums but people don't seem as knowledgeable and you're the only guy that's cut one of these open!

I am curious if you know the reason for the (one way?) inlet port cartridge, to facilitate bleeding perhaps? What would happen if I removed this or drilled through it (the plastic cartridge).

BTW I have rebuild all calipers and changed lines etc, I read your pitfalls pdf too after getting this problem. Brakes were more or less fine before I changed the lines and rebuild calipers. Now I have locking brake!

Looking forward to your reply.
 
jfheath said:
Hi @courtvfr
Welcome to the forum from West Yorkshire. Whereabouts in the UK do you live?

I have seen a photo of a VFR SMC and was surprised at how similar they look - but have no idea whether yours is the same version, or what the internal are like. Broadly speaking, they will be similar, since they do the same job and are manufactured by the same company.

Check again for the relief port in the cartridge. On the one that I showed in the photos 3 posts back, the relief port is almost invisible. I've just gone to recover it from the garage and have taken a better photo:



The compensation port in the cartridge is in the centre of the red circle. It is a very tiny hole - way too small to get a pin through. The top E string of a steel strung guitar will just fit through it - and that is 0.25mm. The pin I just measures is 0.6mm. It is extremely difficult to see unless you know that it is there. Even then it isn't obvious.

The large hole is the inlet port, and it is this that allows fluid to flow into the SMC, between the two piston seals, but the sprung valve - rubber stopper in this case - prevents fluid from flowing back.

There is another tiny compensation port in the bore of the SMC, and this can be seen most easily from the inlet port with the cartridge removed - again, shown in post #64, 3 posts back - circled in red. Both of these ports have to be clear. You cannot see if they are clear, you have to push a fine wire through them. A top E guitar string, or a primus stove pricker. They can become full of hardened gunge from old brake fluid if the fluid isn't flushed out properly and changed every two years at the very longest.

If you have had the cartridge out, make sure that it is replaced the correct way round. On my diagram, the valve / rubber faces outwards towards the fluid line.

You had a rear brake lock up before you started looking at the SMC ? The compensation port is a real possibility, and I would seriously get a hand lens and a good light to try to find the hole in the cartridge. You cannot see it without taking the top or bottom off the cartridge. I've done it only once - on the photo you can see, and it came away cleanly - in fact I would be happy to reuse this one. The blue piece with the spring was the easiest side to get off. The other side having removed the O ring. I think I used a pin down the side as far down as I could get it and then levered it out gently - the pin bit into the side of the plastic. It deformed slightly and lifted out. I didn't care - it was a duff SMC, but in fact it came out quite cleanly.

But if you have read 'avoiding the pitfalls' you will know that there are other reasons for dragging brakes - front pads in the rear caliper, pad spring inserted the wrong way round, corroded pad pins, bent slider pins, too much grease on slider pins, corroded slider pins, pads not seating properly in the caliper at the tab end. Then at the front end - front left caliper dragging will cause the rear to drag. So pads that are too thick for he front caliper (eg adding a heat shield) will cause issues. The most common faults with the SMC is corrosion behind the circlip inside the bore, preventing the SMC piston from clearing the compensation port, an SMC that is no longer circular preventing the piston from moving properly - again, blocking the compensation port. But also, the front caliper has to be free to pivot, so if the needle roller bearings are not moving, it will not be able to move back.



Regarding this question:


I am not familiar with a ball bearing arrangement inside that cartridge in the SMC, but ball bearings are used in one way bleeding valves - I guess it is just an alternative to the little black top hat on mine. But it would not block the compensation port. The port has to be open all of the time when the SMC is not applying pressure - to allow expanding brake fluid an escape route and to allow contracting fluid to pull fresh fluid from the reservoir. Otherwise the brakes lock on.

There is also a compensation port in the master cylinder of the rear brake reservoir. If you try to push the pistons in by hand from the rear caliper (outer pistons), the fluid has to pass through a lot of brake line and through 4 compensation ports (I think). 2 in the SMC (1 in the bore, one in the cartridge) and again in the rear master cylinder. (I think the rear master has a cartridge, but I may be wrong about that - I have never looked).

Be very careful with the cartridge - the gauze is very fragile. In fact the whole thing is very fragile, and it is not listed as a spare component.

The compensation port and the inlet port both get their fluid from same banjo fitting, and the valve will act to stop any backflow from within the SMC bore. I can only hazard a guess as to why it is necessary. It will certainly help when bleeding the system - unless you like to fill the system from the bottom up using a syringe. That valve will stop the fluid from getting beyond the SMC !

But the reason it is there - I don't really know. I have a theory. It is a guess, and as such not worth the paper it is written on. So I'll write my guesswork in italics.

The inlet port to the SMC is also connected directly (via an extremely short hose) to the centre piston in the front left caliper. When the rear caliper outer pistons are activated it is as a result of the movement of the master cyclinder piston and/or the SMC piston. When the brake is released, the piston moves back to where it was - so in theory no fluid has to flow anywhere. Except after the brakes have been applied, the fluid is hotter, so there is a bigger volume of fluid, and it has to go somewhere.
Normally this would release slowly through the compensation port. First the SMC C port, then the master cylinder C port. Both very tiny holes. So I am guessing that in the SMC, if there is no safety valve, it could find a shortcut, and instead of waiting patiently for the master cylinder compensation port to relieve the pressure (up near the battery, past the Proportional Control Valve - which is also releasing pressure), it could return to the centre piston, pushing it out, applying the front brake, applying the SMC and activating the rear brake. ​

All hypothetical, and I don't know the answer. But that valve is there for a reason and it has been carefully thought out, and it works really well. Unless the servicing has been neglected. I certainly wouldn't want to drill it out.
Click to expand...
Just the man I was looking for! I'm down in Berkshire, love West Yorks though.

I had the cartridge out already, but I didn't notice the tiny hole at first!! I was playing with the SMC in the sink and I could tap the outlet port and squirts of water were coming through the tiny hole in the bore, maybe it was blocked before... I'll never know. I've now stabbed a piece of guitar wire through the cartridge, and the gauze. I managed to get the cartridge out of the SMC bore by air hose on the outlet port, that's why I assumed it was a one way valve. Before I took the SMC off, all the other pistons could retract fine, I'm sure the centre one would too but after the first ride it locked and got hot immediately. I'm now going to go over the needle bearings and all other pieces and bleed it quickly tonight, then again tomorrow. I have a pressure bleeder. Maybe air was in the line and expanded and locked the brake-does air expand under heat more so than brake fluid? I did have new pads in too which.... Hopefully I will be able to ride tomorrow!

Your theory in italics sounds good, but the valve has a hole in it to allow fluid back to the circuit. I'm convinced now that it is just a bleeding aid, due to the length of the lines, I might find a junk PCV on ebay to open up and see if there's one in there also as it's a bleed point.... Since it has a tiny 2 way hole then drilling it wouldn't change the fluid flow since it would be limited by the tiny hole in the bore anyway.

Also curiously on the VFR the SMC activates the centre rear piston not the outers... also I was looking at the CB1100xxx blackbird SMC but that unit is mounted to the fork, with the caliper bracket rotating and then activating. Curious why they decided to build our SMC into the caliper bracket, I think that design would be better for bleeding (no need for the 15* angle).
 
jfheath said:
If you can get a guitar string through the hole in the cartridge, it isn't blocked. I am alarmed that you pierced the gauze to do it though. If it was blocked, whatever was blocking it is now floating around inside the cartridge. It will probably be a long thin plug of hardened brake fluid gunge. That may cause problems somewhere down the line.

Before you go out, check that the rear wheel spins. Apply the Smc by hand and check that the rear wheel is locked. Release the SMC and check that the rear wheel is free to move again. It involves getting help, or lying alongside the bike and rotating the wheel with your foot While operating the SMC By hand

When you removed the SMC piston, did you undo the bracket that is on the end of the push rod ? The length of that is absolutely critical.

On the1300, you can check if fluid can return through the compensation port by pushing in the relevant rear pistons by hand - the outer pistons on the ST1300. It is usually too much effort to do it for the full circuit, but opening up the front left centre bleed valve makes it a bit easier. From what you say, It sounds like your plumbing is different so you will have to work that out for yourself.

Are you sure that the SMC operates the centre piston on the rear ?
Click to expand...
Yes the SMC for the VFR activates one piston. Our SMC has a 1/2" bore vs ST1300 14mm bore. I'm guessing being a heavier bike they wanted more rear brake, more often.

I encounter a new problem last night. Following a tip online I wrapped the bleed nipples with ptfe (not covering the hole!) to stop air sucking in when using vaccum bleeder. All well until the PCV and rear caliper. My vaccum bleeder could not get anything out and the plastic bottle imploded. I used the foot brake to bleed and force fluid out the PCV and rear nipples conventionally. While I had fluid in the hose on the PCV bleeder I tested my theory of the cartridge being a bleed valve and it seems to work that way. When I used the foot brake fluid came and the fluid stopped definitely without closing the nipple when releasing the brake pedal.

Not sure if I've caused a different problem. I might video it if I remember. Going to do a 2nd bleed today.

I'll do the pre ride checks and hopefully things are a go.

Oh and I didn't take the SMC piston out I might have meant the caliper piston. I looked at the end under the boot though and put corrosion block grease all over it (ACF) but it didn't look bad for 22years and 60,000mile old.
 
Correct we have the double banjo brake line to the SMC and left caliper.

And yes, it was struggling to draw fluid through the PCV and the rear bleed nipple that goes to the pcv/SMC. Before it always worked but lots of air bubbles from the bleed nipple thread gap,so I wrapped ptfe around. So maybe my vaccum couldn't bring fluid through the tiny hole or wasn't enough to fold the C seal at the end of the SMC plunger as what happens when using the foot pedal or when the line needs extra fluid.
 
jfheath said:
Which model is your vfr ?
Click to expand...
98 vfr800. The front centre bleed worked fine but lots of bubbles even with tape (I pulled 500ml of fluid). The rear caliper has two outer pistons with one outer bleed valve and a centre piston with a centre bleed valve. I've looked at the parts fiche and the it's almost identical. Service diagram attached.

The vtec vfr only has the left front caliper linked to the rear. Same PCV and rear. On the 98-00 vfrs the front calipers have mismatched size pistons meaning one side of the front has more caliper piston volume (diameters and length differences) than the other. Would love to see the math they used to design this.
 

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jfheath said:
Ok - thanks for that. So either there are two proportional control valves, or the PCV has two channels.

Edit - I've just looked at the parts fiche for the only one listed for 1998 - the VFR800FI. There is only one PCV shown. So the diagram that you sent must show the ABS modulator, which I assumed was the PCV - in which case the diagram doesn't show the PCV. Ignore the bit about 2 PCVs if this is the case.


Beware the use of the terms. I have noticed that the term PCV is often used to refer to the bleed valve / nipple which on the ST1300 is located in the area of the PCV unit itself. When I say PCV or proportional control valve, I am referring to the actual pressure releasing unit. If I want to refer to the bleeding point, I will call it the 'PCV bleed valve'.

So on the 98 VFR 800, the front lever operates the two rear outer pistons, but the pressure is controlled by a proportional control valve.
The line to the left front centre piston, also feeds the SMC, which operates the rear centre piston - having passed through a proportional control valve.

(I have to assume that the diagram is correct - this is not always the case in my ST1300 manual)

I have no idea where the PCV bleed valves are in the lines to the rear caliper. I assume that there are two, one for each line, and in each case they will probably be at a high point, somewhere between the PCV and the rear caliper.



The proportional control valve is a device with a straight through flow of fluid which has a chamber which is closed off by a piston, held in place by a strong spring. Under normal braking operation, the spring holds the piston in place. When a predetermined amount of pressure is applied - which would be enough to lock the rear wheel - the spring is overcome and the piston moves back into the chamber, releasing some pressure from the brake line, thus preventing lockup of the rear wheel. So from the diagram there is one of these devices for each of the two lines to the rear caliper.

Back to the plot.

Note - anyone reading this - @courtvfr 's issue is with the system on a 1998 VFR800, which is very similar to the ST1300, but it is not identical.
If you have fluid coming out of the front left centre piston when you bleed, the line up to the inlet port of the SMC is clear.
If you cannot get fluid out through the PCV bleed valve, then that suggests a blockage in the SMC, or in the line to the PCV bleed valve, or in the PCV itself.
What happens at the rear caliper centre piston is irrelevant at this stage - that is further down the same line, so if fluid isn't getting to the PCV bleed valve, then it won't get to the rear centre piston either.

Just out of interest - are you able to draw fluid through the rear outer pistons ? (which are also fed directly from the rear reservoir and rear master cylinder). I assume that you can, because you can get fluid out of the front left centre piston.

The flow of fluid from the SMC to the PCV bleed valve is not usually a problem, but it is not impossible.

What I do not know, and I cannot tell you, is how you know which PCV unit is the one for the rear centre piston (ie the one that is fed by the SMC.
So (assuming that there are two bleed valves for each of the two lines to the rear caliper) - how do you know which bleed valve to operate ? I don't need to know the answer, but you do.

The other thought is that you may have compounded the original problem if you put that cartridge into the SMC the wrong way round. If it was upside down, the valve would stop fluid flowing in the way that it should. Applying more vacuum would simply lock that ball bearing against the inlet port. The only flow of fluid would be through the tiny compensation port.

There is a way that you could test whether the fault is with the cartridge. It will cost you a lot of fluid though. That is - when you take the cartridge out to check whether it is the right way round - if it was correct, then leave it out, seal it up and fill the system. If you can now draw fluid through at the rear centre piston and at the corresponding bleed valve on the line to the rear centre piston, then the fault is with the cartridge or its installation. If you can't then there is a blockage in the line or in the PCV unit itself.

In theory, no matter what position the piston is in inside the SMC, the fluid should be able to flow past the primary seal from the inlet port and up to the PCV bleed valve. In practice it may be jammed tight, or the primary seal may have inverted itself (goodness knows how that could have happened). The primary seal yields easily from even slight pressure from behind.

Right - breakfast time, walk and shop (away from the madding crowds).

I may be some time.
Click to expand...
There's no ABS or 2nd PCV. The other component in the loop is either the delay valve (front fork leg to prevent front brakes engaging before the rear) or the rear master cylinder. Definitely no 2nd PCV!

Definitely put the cartridge in the right way, as it forces fluid through when the foot brake is applied.
I was thinking of removing the cartridge to test.... I only have maybe 2.5L of fluid left (had a 4L container). Been pumping so much through it's insane! Did I mention I changed the hoses which is what caused this bleed scenario.
Have you used PTFE tape around the bleed nipples to prevent air bubbles?
I took a video of the one way 'feature' :
 
Recently replaced my SMC with a new unit. I had no adverse braking symptoms, but did have a "clicking" when manually compressing the SMC piston while mounted on the caliper. That's what prompted the changeout.

Out of curiosity, I dismantled the old unit to see what conditions prevailed inside. (I did my CSC recently and it was a mess!).

The SMC was in quite good shape compared to the CSC. The plunger seals looked clean and intact and the cylinder barrel showed no signs of scoring, corrosion or wear. There were no deposits inside (unlike the CSC), just traces of hydraulic fluid. The ports were clear, although i couldn't see if there were any blockages inside them. The external boot was intact.

20200904_101502.jpg 20200905_093949.jpg 20200905_094040.jpg
 
An update. This last week I have been uncovering 2 years worth of 'neglect'. ie I have ridden my bike only half as much as I would ride normally. My max 18 months before changing brake fluid (I normally do it every winter) seems to have extended to 2 years. So it MUST be done. I had intended just a simply pump out the old flush with new and then pump out the new with brand new and bleed. Except - when I took off the left front caliper - it wasn't moving very easily on the slider pins. When it did, it moved back. I didn't know if it was air pressure or because of that very short hose between the centre piston and the SMC inlet port. So I had to investigate.

Whle I was at it. I popped out each of the pistons in turn. Pristine. Bores also pristine. No gunk or slime anywhere. Which is what I would expect.

1710671589192.pngI decided to pop out the inlet port cartridge and check that. A gently blast of air from the outlet port lifted that out easily enough. That proved a couple of things.

1) The primary seal was working OK as none of the air came out from the inlet port.
2) The compensation port was clear, as a fine but powerful jet of air from there was enought to lift the cartridge out of its bore.

Easy enough to take off the blue cover - small tab first - being careful of the spring and seal in there, and this was clean enough.
Just a tiny bit of dirt in the corners of the moulding of the white cartridge. The gauze was clean.

SO I went looking for the tiny compenastion/relief port in the white cartridge. The one that is arrowed in the diagram. Now I know this is difficult to see, so I took it to me desk with a 10x iluminated magnifying lens. Yes, that is where it is supposed to be - there was an indentation. But it didn't look like a hole. Where's my top E guitar string that I snipped off Larry's Guitar machine head ?

Got it. The wire wouldn't make any progress. Primus stove pricker - which I had used successfully on my other SMCs - far too thick.
So I got a pin to make the indentation a bit larger and to guide the wire into what I thought was the hole, a pair of pliers and started to push the end of the guitar string forcefully into the 'hole'.

A LOT of resistance, so I started twisting. At the other end, I couldn't see a hole at all - although I could see where the start of it should be. I persevered with twisting the E string. About 3/4 of the way in, it became easier but I had to push through the last bit - drilling a way through with a very blunt guitar string.

Once it was through I pulled it in and out repeatedly until the resistance could be managed without pliers. This is how it had felt the last time that I took one out.

Now - just to be clear about this. The compensation port in the cylinder bore was perfectly clear. No gunge or grime came out of the port in the white nylon block.
The rear brake had always been releasing perfectly well, and under tests, getting the rear brakes hot just by braking hard with the front brakes, they would cool off reasonably quickly.
I know that nylon compensation port is tiny, but the E string and primus pricker have always fitted. I didn't have any thinner guitar strings to try it (and if I had, I wouldn't have had much of my fingers left), but I feel that the nylon port was blocked with something - or never drilled and reamed properly in the first place.

But - ever since fitting this new SMC as a precaution 30,000 miles ago, pushing in the rear pistons has been difficult. I'm wondering if the nylon compensation port has always been blocked. In which case, fluid must have been escaping under that tiny sprung rubber seal inside the cartridge. There must have been a hole - I cannot believe that I drilled a perfectly straight hole from scracth with a thin steel guitar string. But it was either very narrow or blocked. Or both.

I'll go and refit it and report back on the ease with which the rear outer pistons can be returned with thumb pressure. It won't be today though. Still got lots of cleaning up to do.
 
Last edited:
jfheath said:
An update. This last week I have been uncovering 2 years worth of 'neglect'. ie I have ridden my bike only half as much as I would ride normally. My max 18 months before changing brake fluid (I normally do it every winter) seems to have extended to 2 years. So it MUST be done. I had intended just a simply pump out the old flush with new and then pump out the new with brand new and bleed. Except - when I took off the left front caliper - it wasn't moving very easily on the slider pins. When it did, it moved back. I didn't know if it was air pressure or because of that very short hose between the centre piston and the SMC inlet port. So I had to investigate.

Whle I was at it. I popped out each of the pistons in turn. Pristine. Bores also pristine. No gunk or slime anywhere. Which is what I would expect.

1710671589192.pngI decided to pop out the inlet port cartridge and check that. A gently blast of air from the outlet port lifted that out easily enough. That proved a couple of things.

1) The primary seal was working OK as none of the air came out from the inlet port.
2) The compensation port was clear, as a fine but powerful jet of air from there was enought to lift the cartridge out of its bore.

Easy enough to take off the blue cover - small tab first - being careful of the spring and seal in there, and this was clean enough.
Just a tiny bit of dirt in the corners of the moulding of the white cartridge. The gauze was clean.

SO I went looking for the tiny compenastion/relief port in the white cartridge. The one that is arrowed in the diagram. Now I know this is difficult to see, so I took it to me desk with a 10x iluminated magnifying lens. Yes, that is where it is supposed to be - there was an indentation. But it didn't look like a hole. Where's my top E guitar string that I snipped off Larry's Guitar machine head ?

Got it. The wire wouldn't make any progress. Primus stove pricker - which I had used successfully on my other SMCs - far too thick.
So I got a pin to make the indentation a bit larger and to guide the wire into what I thought was the hole, a pair of pliers and started to push the end of the guitar string forcefully into the 'hole'.

A LOT of resistance, so I started twisting. At the other end, I couldn't see a hole at all - although I could see where the start of it should be. I persevered with twisting the E string. About 3/4 of the way in, it became easier but I had to push through the last bit - drilling a way through with a very blunt guitar string.

Once it was through I pulled it in and out repeatedly until the resistance could be managed without pliers. This is how it had felt the last time that I took one out.

Now - just to be clear about this. The compensation port in the cylinder bore was perfectly clear. No gunge or grime came out of the port in the white nylon block.
The rear brake had always been releasing perfectly well, and under tests, getting the rear brakes hot just by braking hard with the front brakes, they would cool off reasonably quickly.
I know that nylon compensation port is tiny, but the E string and primus pricker have always fitted. I didn't have any thinner guitar strings to try it (and if I had, I wouldn't have had much of my fingers left), but I feel that the nylon port was blocked with something - or never drilled and reamed properly in the first place.

But - ever since fitting this new SMC as a precaution 30,000 miles ago, pushing in the rear pistons has been difficult. I'm wondering if the nylon compensation port has always been blocked. In which case, fluid must have been escaping under that tiny sprung rubber seal inside the cartridge. There must have been a hole - I cannot believe that I drilled a perfectly straight hole from scracth with a thin steel guitar string. But it was either very narrow or blocked. Or both.

I'll go and refit it and report back on the ease with which the rear outer pistons can be returned with thumb pressure. It won't be today though. Still got lots of cleaning up to do.
Click to expand...
Thanks for the update John.
Most high E strings won’t fit unless they are very light set from an electric guitar.
The only thing I have found that fits is an acupuncture needle.
The hole IS that small.
I have found several cartridges that appeared clean, only to find that small hole blocked.
 
Igofar said:
Thanks for the update John.
Most high E strings won’t fit unless they are very light set from an electric guitar.
The only thing I have found that fits is an acupuncture needle.
The hole IS that small.
I have found several cartridges that appeared clean, only to find that small hole blocked.
Click to expand...

I was comparing like for like. I have a cartridge on the 'spare that I replaced after 37,000 miles, suspecting it was the cause of an issue. It wasn't - so it is dismantled, cleaned up and sealed it away as my next spare. (Actually, I need a service kit for that spare unit because I damaged the circlip trying to see if it was actually possible to remove it while it was still on the bike. So that would be a 'no' then !) When I got my new Taylor 724ce, first thing I did was to put on some lighter, mellower strings. Top E is 0.012 ins. Yes the electric ones are a tad thinner (0.009). But I checked my 0.012inch top E in the removed SMC cartridge. It was a nice fit. Went in and dragged slightly all the way through. So I packed that away with all of my brake stuff for future use (ie today).

Your experience with that port in the nylon block was the reason I wanted to remove it and check it out. Nothing was pushed out when I eventually got the E string through.

I've just finished today in the garage. It is taking me ages compared to 2 years ago. I'm feeling my age I think. But I got the fluid into the rear system and yes, it has improved the ability to push the pistons back in - although I haven't even attempted to bleed it properly. Just went round out of sequence from the back simply to get fluid into the lines. I wanted fluid in the calipers and SMC rather than having them sitting there in an open system. Let the new fluid dissolve any contamination, and I'll bleed it all out next week.
 
MidLife said:
Too late now, but wondering if there would have been a way to also air test that prior to blowing the cartridge out?
Click to expand...

After the block was out, I blasted air through form the outlet port. I was very surprised by the strength of the jet of air. The air would have been prevented from coming out through the inlet port - by the primary seal, so it must have been entirely through the compensation part in the cyclinder wall. In fact I checked that and could feel the blast on my finger from the tiny hole, and nothing from the larger hole.

That doesn't answer your question. To push out the cartridge, I was using very short blasts of air - because if the cartridge was stuck, the gauze would have taken the full blast. I didn't want to damage it. However, I was holding my finger just above the cartridge - so that it didn't take flight across the garage. But I didn't feel any jet of air coming through. But I wasn't checking for it - my plan was to get out my top E guitar string to check the hole - safely put away for just that purpose.

But yes - if the hole is clear, I guess that you should be able to feel it. That is worth checking out on my spare setup at some point. Actually, I'll use the very old cartridge - remember the diagram of the sawn down SMC that I posted years back ? I still have the cartridge from that - I took it out and cleaned it up and decided it was a handy spare to have.
 
If we're down to which gauge high-e string to make a difference, that is TIGHT.
It's a tiny string, but I guess the more appropriate adjective should be "teeny- tiny."
A matter of degrees (ok, then, thousandths), but they apparently matter.
So, if you only have cello strings or bass strings... don't even bother.
 
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