Braking issues

[Johnny24]

Good morning @Johnny24 !

I bet it is bitterly cold over there in Scarborough.

Just to be certain - are you talking about the inlet port - the one that is circled in red on this picture of a new SMC ??

View attachment 251600

That cover is secured by 2 bolts that can be seen, and is sealed with a rubber O ring.

Beneath that is a very important mechanism which has a very find white gauze. It is probably the gauze that you can see. I'll post some pictures - but in the mean time, do not go probing it. It is a filter cartridge and a one way valve, and is safety critical !!!

Yes -t he tough white plastic plugs have to be removed before fitting. They are, as you rightly surmised, in place to keep the dust out.

I'll be back with the pics.

Here you go. First with the black cover removed, showing the O ring and the gauze filter.

View attachment 251603

And here showing both sides of the filter.

View attachment 251605View attachment 251606

And here showing the filter taken apart.

View attachment 251607

This removed from a dead SMC. Its not advisable to remove it, as it is very delicate and not easy to do without causing damage. And since you cannot buy them separately.....

The purpose ?

Its purpose is two fold. The inlet port allows fluid into the SMC chamber to provide a constant supply to the rear outer brake pistons. When the SMC is released, the device shuts off the inlet port so that fluid from the SMC cannot be pumped back up the brake lines. The gauze is there (there are two circular gauze filters - the white circular one on the left - the one you can see once the white plug has been removed, and the one with the blue cap which also supports a spring). Both stop any debris from entering the SMC piston bore and also interfering with that one way valve seal.

All brake system master cylinders have a compensation port. This is a very very tiny hole which allows the relief of pressure from the brake lines as the pistons in the caliper return, and also to allow for expansion or contraction of the fluid as the temperature increases or decreases. If fluid cannot escape through that tiny hole, the brakes can remain locked on. (Nothing to be alarmed about by that statement, that is how the brakes work).

However, with a normal brake lever, if the brakes did not release properly, you would be aware of it instantly. In the case of the SMC, you do not have the 'feel' at the brake lever or the brake pedal. So if brake fluid is not changed at least every two years (I do mine every year), then problems can begin to form and the rider would not necessarily be aware of the problem developing . And the SMC has 2 compensation ports - one in the side of the SMC bore, one just behind it in that plastic housing - by-passing the one way valve.

Is it a concern ? No. I've ridden 170,000 miles on bikes with SMCs, without issues. It's knowing how to service - and why - that I think makes the difference.

Scarborough to Ilkley is a pretty good ride - we often do it - if you ever want to come and pick my brains. (Pretty lean pickings on some days though !).

It's certainly bitterly cold in my garage which is where I've been most of the weekend!

Thank you for the quick, full and helpful reply. Yes, I was indeed talking about the inlet valve on the SMC. After I'd posted my query, I looked more closely at the old SMC and was able to see the white gauze at the bottom of the inlet, and could also see that as I worked the plunger on the cylinder some remnants of the old brake fluid came up through the gauze which a) reassured me that it was not impermeable b) indicated it was supposed to be there and c) stopped me from poking around at the new one. Anyway, the new one is all fitted now, slider pins cleaned up and re-greased and the next job is to put in some fresh brake fluid and hope it all works.

I understood from one of your other posts (Going on an Air Hunt) that there was a compensation port at the bottom of the SMC inlet but I hadn't appreciated that there was a membrane/gauze as you have described.

The bike is having various jobs done at the moment as well as sorting out the brakes. Once all the plastic comes off it's sometimes difficult to stop! It all needs to be shipshape as I'm off to the continent in two weeks (coronavirus allowing); North Sea Ferries to Zeebrugge, down to Perpignan, along the N260 on the southern side of the Pyrenees to the Picos de Europa, then back from Santander to Portsmouth and home.

Send me an email next time you come over to Scarborough if you feel like meeting up for a coffee.

 

[krist0ph3r]

It all needs to be shipshape as I'm off to the continent in two weeks (coronavirus allowing); North Sea Ferries to Zeebrugge, down to Perpignan, along the N260 on the southern side of the Pyrenees to the Picos de Europa, then back from Santander to Portsmouth and home.

Nice! I'm planning my first ride to the continent in August (hope the virus blows off by then!) - Belfast to Cairnryan by ferry, Cairnryan - Hull, Hull to Rotterdam by ferry, Rotterdam - Ameland (with another short ferry on the way ) and back the same way. Sounds like more ferry and less ride, but that's cos it's my first time and I'm nervous about riding on the other side of the road

 

[Johnny24]

Good stuff. That sounds like an excellent ride. We have done a similar route in reverse - from Santander, in the Picos and then along the 260 and back up through France. I'm jealous - we haven't been back for about 6 years now. From the maps it looks like they have a new road between Jaca and Pamplona - which would bypass some of the nice flowing twisties of the old 240 and 330 roads. I suppose the higher Pyrenees will still be closed.

I just wanted to focus on chasing bubbles around in the air hunt document. Getting at un-documented filters was probably a bit of information too far for that purpose.

Thanks for the email address. I've made a note. You might want to edit it out of your post - since it is available from anywhere !

Having fitted the new SMC, and new pads in front and rear, I am at the end of my tether trying to bleed the system. I don't know what is going on. I've not had problems like this previously. I've now done the whole procedure twice. I use a vacuum pump and non-return valve. Weirdly - and this is what I can't understand - the rear brake pedal doesn't feel too bad (I thought if I was going to have problems it would be here as a result of installing the new SMC) and I reckon one more run through the process should see it firm up to an acceptable level.

The problem is with the front brake lever. I've now gone through stages 1 and 2 of the bleed process (front lever and reservoir, top - outer - bleed valves) about five times. I'm still getting copious amounts of air being drawn through. Not just a few bubbles, but as much air as brake fluid. And it's not getting any better. I can't fathom where it's coming from. I have a feeling I'm doing something really stupid but I have no idea what it is. I wondered whether I have a split in a brake line somewhere, perhaps from removing the front left caliper, but I can't see any leaks. The front calipers should be the easy bit to bleed as the circuit has no connection (I think) to the circuit from the rear reservoir and should be pretty straightforward.

I have completely run out of ideas. Help!

 

[Uncle Phil]

I have completely run out of ideas. Help!

Check to see if one of your bleeder nipples is leaking. It does not have to leak much to cause your problem.
Put some teflon thread tape on the threads (only a wrap or two).
I've been told that they slightly deform over time depending on how many times they have been tightened.

 

[Andrew Shadow]

Johnny24;
Have you tried bleeding the front brake master cylinder directly at the banjo fitting where the brake line attaches to the master cylinder?
If a lot of air was introduced in to the system you sometimes can not get the air out of the master cylinder without bleeding it directly. Once you have purged all of the air from the master cylinder it is usually much easier to bleed the rest of the system.

 

[Johnny24]

Once you are certain that the system has drawn enough fluid through, ditch the vacuum bleed valve and use the good old tube method- or better still one with the non return valve.

The good thing about vacuum pumps is that they get a good amount of fluid through, while you go round dislodging the air.
The bad thing is that air is drawn from the outside of the bleed valve, past the threads and it come out looking like air in the line.
You cannot get a good feel with just the vaccum pump, in my experience. You can only get close to it.


With the vaccum pump, sucking away, bash and flex the tubes and tap the metal joints. Also the union at the reservoir. Some find it useful to crack open the bolt to allow air to escape. Mine feel perfect without ever having to disturb that joint. But it may be necessary to turn the bars left to right. (Fill the reservoir and put the cap on before doing this though). It helps shift any air in the union, along with a bit of tapping. You can get some of it to bubble up through the compensation port.

With the bleed tube. Valve closed. Pump fast a few times, hold in the lever. Open the valve until the fluid nearly stops moving. Close the valve. Repeat.

Also leave the lever tied back to the bars when you are about done. Leave it overnight.

Just bleed the top bleed valves on both calipers left and right, with the front lever. No need to involve anything else to get the front solid. The lower bleed valve on the front caliper (for the centre piston) is part of the rear circuit - nothing to do with the front lever.

Get the front rock solid and then do steps 3 to 7 from the rear reservoir. Those two systems are completely separate.

Thanks everyone for the suggestions.

I wrapped the threads on the bleeders with PTFE tape. This made a big difference to the amount of air seen in the bleeder tube. It didn't seem to make any difference to the amount of air in the system as I still had a really squishy brake lever.

I then tied the brake lever right back to the handlebar and left it for 24 hours, occasionally returning to the bike and tapping and wriggling hoses and swinging the handlebars from side to side and bumping them up against the stops. I was astonished to find 24 hours later that this had completely transformed the feel at the lever, and it is now as firm as it has ever been. I'm not sure why this has worked. I'm not a physicist ( I failed my Physics "O" level) and don't know anything about fluid mechanics. But the following is what I think has happened (with thanks to John Heath's article "Going on an Air Hunt").

Brake fluid is not compressible, but any air in the system is. When the brake lever is tied back it exerts pressure on the system. That pressure cannot squash the fluid, but it does squash the air. Bubbles (ie air) in the system must therefore be made smaller. If they have less surface area then presumably there will be less surface tension. I think that means they are less able to hold onto the sides of the brake lines and are then freed into the fluid and float very slowly up to the highest point they can reach. The bubbles are likely to come up to the joint under the reservoir and can then escape into the reservoir and freedom once the pressure on the lever is removed.

When I was about 9 I followed the instructions from a comic, possibly the Eagle (remember that?) to make a toy. It consisted of a used bottle of lemonade - one that had a rubber screw top. The bottle had to be filled right to the brim with water, so that there was no air under the screw top. A used matchstick was put in the bottle. Then when you tightened the top right down the matchstick slowly sank to the bottom. To make the matchstick rise you simply released the top very slightly. With practice you could get it to hover at any level you chose. (Life was simpler in those days). I never understood why that worked either - but I think now the increased pressure reduced the volume of the tiny air bubbles on the matchstick, and smaller bubbles mean less buoyancy, so the matchstick sinks. And vice versa. Similar to the air in the brake hoses.

I think it has worked for the front brake lever because the reservoir is the highest point in the FRONT system. It wouldn't necessarily work for the REAR because (as John has pointed out) the PCV is not the highest point in the rear system.

This may all be complete nonsense. But I don't really care. I now have brakes that work!

 

[Georgevn]

PS Is the caliper stopper bolt properly seated onto the caliper holder/bracket?

Yep, I did that once and the rear wheel locked solid. Double check that the bolt actually goes into the hole on the caliper stay. Very easy to get wrong!

 

[Johnny24]

The reduced surface tension of the squashed air bubble theory is the only explanation that I have come up with. And I must admit that playing with a plastic deep sea diver that came from a cornflakes packet in a screw top bottle was great fun. But I worked out how it worked - which I was really proud about as a kid. The memory did pop into my head as I was writing the air hunt stuff.

It will work for the rear brake circuit, but I reckon those tiny bubbles collect together to form big bubbles in the air traps. But you know where those are, and you can then chase the bubbles round to the nearest valve.

Well done for sorting it.

Thanks for all the help and suggestions.

 

[ChriSTian_64]

With the wheels off and pads removed, carefully extend the caliper pistons with the lever/pedal. A block of wood in the caliper will keep them from going too far out. Note that the pistons have no stop, so you can push them all the way out if you're not careful. Then, using a flat shoestring soaked in brake fluid, wrap it around the pistons and clean the outer surface. Then use a c-clamp to gently and slowly squeeze the pistons back in, a step at a time. This is about the best you can do without taking anything apart, outside of wheel and pad removal.

I have to try that. Just noticed that my rear brakes are slightly squeezing the disc.
On center stand, when I'm turning the wheel with my hand, it stops after half a turn, or less.
It is an ST1100, non ABS, year 2000.
Just read that topic from the beginning. Mine is not as severe... well not yet.
I checked the rear brake oil reservoir, and it is a bit a darker gold than new dot4 : it is actually the same color as Honey.
Anyway, I will bleed the front, the rear, and while the wheel will be removed to change the tire, I will inspect those caliper pistons and do them the "shoestring trick".

Can you believe the rear tire was changed only once on that bike : in 2002. Yeap ! A long time ago.
So I guess those brake pistons are in need of some attention.

There is something I don't get : when we release the brake, the pistons should get back to their normal position, no ?
We can clean them because they are still out. Why ?
Hum... because they are a bit dirty and that is why they need cleaning ? A clean piston would normaly get in after we release the brake, right ?

 

[OkieRider]

The brake pistons go out because of hydraulic pressure They retract only a little bit due to elasticity in the seals and some extra kick from runout in the rotor. You don't want the pistons to retract a lot or, as your pads began to wear, you would end up having to pump your brakes up every use. A clean piston would normally go in just a little bit so it cleared the rotor.

 

[jfheath]

Each time you apply the brakes, the pistons move out a tad to press on the brake disc rotor.
The pads wear a very tiny bit each time.
The next time you apply the brakes, the pistons have to move that tiny bit further because some of the friction material was warn away last time. This distorts the seals a bit, but the pistons also slide past the seals a bit.
But when released, they only move back by the amount the seals can pull them back. (And maybe a bit more as @OkieRider said, from being pushed by disc runout.)

So Over time, the pistons creep out a little more each time as the pads wear down. The action at the master cylinder allows extra fluid into the lines from the reservoir, and allows surplus fluid to return through that tiny compensation port at the bottom of the reservoir. That way, you never have to adjust the brakes as the pads wear, as you have to with cable operated brakes. But you do have to make sure the reservoir is full when you fit new pads so that it does not run dry before the pads wear out.

 

[Upt' North]

And make sure it isn't overfull when you push the pistons back in after cleaning.
When you pump the pistons out place a piece of wood or plastic to prevent them coming out too far and they won't come out together, so when one touches the wood the other will then follow, if you follow. It'll make life easier, possibly.
Upt'North.

 

[Flexit]

Thanks everyone for the suggestions.

I then tied the brake lever right back to the handlebar and left it for 24 hours, occasionally returning to the bike and tapping and wriggling hoses and swinging the handlebars from side to side and bumping them up against the stops. I was astonished to find 24 hours later that this had completely transformed the feel at the lever, and it is now as firm as it has ever been.volume of the tiny air bubbles on the matchstick, and smaller bubbles mean less buoyancy, so the matchstick sinks. And vice versa. Similar to the air in the brake hoses.

I do this all the time. My sidecar is quickly detachable with a snap connector for the sidecar brake (which is operated off the front brake). When I refit the sidecar there is always a tiny bit of air released into the system so I use the tie back the brake lever system to save me having to physically bleed the brakes each time. First came across this on a trip across the english channel. My front/sidecar brake had always been a bit soft which I had put down to the extra brake and extension... anyway, the crossing was due to be fairly rough so one of the deckhands (DFDS ferries.. always bike friendly) as well as chocking the wheels grabbed a piece of rope and tied the brake lever back as an additional precaution. Came back to the bike two hours later and the brakes were sooooo sharp.