ST1100 Rear Caliper Sticking

[cam_1100]

I know sticky rear calipers seem to be a common thread, and I've gotten a lot of information from these forums so far, but I wanted to run my situation by the experts here and see if I understand things correctly or if I may be missing something.

First, some background:
I am working on a borrowed '01 ST1100 ABS. A friend gave it to me on a long-term/indefinite loan to try out as a two-up platform for me and my daughter in exchange for going over it and getting it working right again after sitting in her garage for about 7 years. I haven't ridden it yet, it went from her garage to my garage by trailer late last fall and has been waiting for me to get time to work on it. Among several unrelated things I've done to it so far, I flushed the brakes lines (the old fluid was REALLY dark and dirty), bled the system following the procedure in the manual and realized that the rear brakes were not releasing (which made it very hard to move around the garage). I took apart the rear caliper, cleaned everything and replaced the seals with OEM parts, and replaced the pad slide pin even though it wasn't in bad shape, also with a new OEM part. The pistons had a little bit of dirt/corrosion on them, but cleaned up easily. I got everything back together and refilled/bled the brake lines and the rear caliper is still not releasing.

The symptoms:
They are not exactly locked up because I can move the wheel with a lot of effort, but it's way more than just dragging. I took the rear caliper off again but left the brake lines connected. I can press in the center piston relatively easily by hand, so this tells me the rear brake system is okay. I can move the outer pistons each by hand, but pushing in one pushes out the other. This tells me the pistons are moving freely. But when I try to push them in together with a C-clamp and a wrench across both outer pistons they will not budge. So looking at the brake system diagrams, it seems to me that the problem may lie in the SMC (or possibly the PCV though I'm not really familiar yet with how that works). I've read here about the pinhole-sized release port in the SMC getting blocked and causing this problem, and it would seem to match what I'm seeing.

The questions:
All I've done is bleed the brakes, the bike hasn't been ridden. Will either the front or rear brake systems activate the SMC if the bike is not moving?
Is there a way to test for sure if the SMC is the culprit? I've thought about letting pressure out of the secondary system to see if this frees up the outer pistons, but I'm not sure if this effectively isolates the SMC.
Is it worth trying to clean/rebuild the SMC?
If replacing the SMC, should I go straight for a new unit or take a chance on a used one? So far I've only found a single used one on e-bay for $160.
Where is the best place for getting a new SMC? Searches from several suppliers show a new part going for around $240+.

As you can probably tell, I'm leaning towards replacing the SMC with a new one, but at that price I want to be sure it's the correct fix. Is there anything else I could have possibly missed?

Thanks in advance for any advice.

 

[cam_1100]

Thanks for the tips and the excellent article, John.

While your article confirmed my basic understanding of how master cylinders work as isolated units, my questions about the SMC were more how it specifically affects the rear brake caliper in this system and if the compensation port being disabled in some way would be the most likely (only?) scenario to produce what I am seeing. I think this is true although WHY the compensation port is disabled (blockage, stuck piston, etc) has yet to be determined. As you suggested, starting with checking the mechanical linkage to see if it is allowing the piston in the SMC to return properly is probably the best place to start, then digging into the SMC itself from there. I didn't get a chance to look at it last night, but I hope to have some time tonight to check it out.

The other question I had that still isn't clear to me is if the SMC can be activated without the bike moving (and thus no torque applied to the front left caliper to operate the mechanical link to the SMC). From the diagrams I've seen and the explanation in your article, if the supply of fluid from the rear brake system is essentially used as the "reservoir" for the SMC, pressurized fluid from the rear brake would have a direct path through the compensating port of the SMC to the rear caliper, bypassing the piston in the SMC. However, this makes me question how the system after the SMC was pressurized in the first place if the compensation port is blocked in some way.

At any rate, I'll try to take a better look at the SMC soon to check for any obvious issues and better understand how the entire system is supposed to function.

 

[cam_1100]

Your explanation makes sense. Diagrams aren't necessary (I have the ones from your article anyway!). In my mind, the primary seal on the piston forms a cone inside the bore of the cylinder. Fluid can flow towards the caliper simply by collapsing the walls of the cone inward. Fluid flow is prevented in the opposite direction because it pushes the base of the cone against the wall of the cylinder, sealing it off. And in fact, the more pressure applied in this direction, the tighter the seal. That may not be entirely accurate, but I think it works as a simple model.

So based on your article and the subsequent comments, and your explanations here I think I understand how pressure is being applied to the outer rear caliper pistons in a situation where 1) the bike has not moved (no activation of the linkage connected to the front brake), and 2) the compensation port in the SMC is blocked:

• Pressure is applied at the rear brake pedal
• Fluid flows through the delay valve
• Fluid flows into the SMC cylinder through the "recupirating" port (the larger port on the SMC)
• Fluid bypasses the primary seal (because of the valve action you described)
• This pressurized fluid forces the outer pistons of the rear caliper outwards
• The calipers are prevented from returning because the blocked compensating port and the (properly behaving) primary seal prevent fluid from returning from the caliper through the SMC.

This is where I am now. My plan to narrow this down is to:

• Relieve pressure on the input side of the SMC. The rear caliper pistons should remain stuck. Otherwise, it indicates a problem before the SMC.
• Relieve pressure on the output of the SMC. This should allow me to return the caliper pistons to the fully seated position. Otherwise, there is a problem past the SMC.
• Manually activate the SMC linkage. This will verify the linkage is not seized. If this also pressurizes the outer rear caliper pistons, it verifies the piston in the SMC is not stuck.
• Assuming the rear brake is again stuck at this point the problem is either a blocked compensation port or the SMC piston not fully returning. Either way, a repair/replacement of the SMC is in order.

I appreciate you taking the time to help me work this out in my head. It's been a big help. I think I have enough now to keep me busy investigating this problem for a while. I'll update with what I find.

Thanks again!

 

[cam_1100]

Didn't get much chance to look at things last night as storms knocked out power all evening...my garage was a black hole from 6pm-midnight. On my way into the house after work I did try to move the front left caliper by hand. I couldn't really move it at all; a "slight wiggle" is about all I could produce. We lost power soon after so I didn't make any progress beyond that. Before I disconnect any hoses or open and bleeders tonight, I'll try to verify if the linkage isn't seized and work my way up to trying to directly operate the piston in the SMC.

 

[cam_1100]

Finally got some time to look at this some more. I broke loose the link plate bolts on the SMC linkage and then I was able to move the caliper and actuate the SMC plunger. I'm guessing the linkage was okay and that the forces under normal braking would have been enough to operate the SMC, I just wasn't able to brake the "crud stiction" by hand.

I then proceeded to follow my plan listed above. I opened the lower bleed valve on the left front caliper. This had no effect on the rear caliper pistons. I then opened the banjo bolt on the output of the SMC and this allowed me to press in the rear caliper pistons, as expected. This pointed to an issue in the SMC itself. So I removed the SMC, disassembled it, and found that it was full of varnished brake fluid and other debris. This is the input port:


The cylinder was also full of crud. I removed the plate over the input to get to the filter (the greenish thing at the bottom of the port) and access the fluid ports underneath. I took apart the filter and cleaned it as much as possible. I then used some brake cleaner and various picks and wires to clean out the compensating port...describing it as a "pinhole" is generous as I've never seen a pin that small. I ended up using a single strand from some braided speaker wire to clean it out. After getting the rest of the parts cleaned up, I coated everything with fresh brake fluid and put it all back together. At this point everything is back on the bike, but I haven't refilled or bled the brake lines yet so I'm not sure if this fix will work. If not, I guess I'll spring for a new SMC. But at the current cost of new units, and the lack of used ones available, I figured a rebuild of this one was worth a shot.

I'll update again when I get a chance to test the brakes.

 

[fnmag]

Finally got some time to look at this some more. I broke loose the link plate bolts on the SMC linkage and then I was able to move the caliper and actuate the SMC plunger. I'm guessing the linkage was okay and that the forces under normal braking would have been enough to operate the SMC, I just wasn't able to brake the "crud stiction" by hand.

I then proceeded to follow my plan listed above. I opened the lower bleed valve on the left front caliper. This had no effect on the rear caliper pistons. I then opened the banjo bolt on the output of the SMC and this allowed me to press in the rear caliper pistons, as expected. This pointed to an issue in the SMC itself. So I removed the SMC, disassembled it, and found that it was full of varnished brake fluid and other debris. This is the input port:


The cylinder was also full of crud. I removed the plate over the input to get to the filter (the greenish thing at the bottom of the port) and access the fluid ports underneath. I took apart the filter and cleaned it as much as possible. I then used some brake cleaner and various picks and wires to clean out the compensating port...describing it as a "pinhole" is generous as I've never seen a pin that small. I ended up using a single strand from some braided speaker wire to clean it out. After getting the rest of the parts cleaned up, I coated everything with fresh brake fluid and put it all back together. At this point everything is back on the bike, but I haven't refilled or bled the brake lines yet so I'm not sure if this fix will work. If not, I guess I'll spring for a new SMC. But at the current cost of new units, and the lack of used ones available, I figured a rebuild of this one was worth a shot.

I'll update again when I get a chance to test the brakes.

I hope your cleaning sorts out the problem.
That SMC needs to be spotless on the inside.

 

[cam_1100]

The problem now might be whether something similar is happening at the rear brake pedal master cylinder. ... You may want to consider taking a look at that at some stage

Yes, I had the same thought but decided to leave well enough alone for now. I will add cleaning rebuilding the remaining master cylinders to the long-term repair list.

It is still worth taking the two linkage plates off and taking out the 3 hole casting to see how freely those needle roller bearings move. ... The movement will likely be minute ... Working it with your hand should be enough.

I did clean and grease the needle bearings and all the bushings in the linkage. The bearings don't roll all that freely as the range of movement is maybe 10-15 degrees of rotation so they only need to relieve a small amount of friction as the bushings rotate on the mounting bolts. The bearings themselves were pretty clean to begin with, but just cleaning all the accumulated dirt and grime from the linkage and all the pivot points seemed to help a lot. I can now easily torque the front caliper enough to actuate the SMC.

I was very happy with the end result after cleaning the SMC and the action of the brake linkage seems much better now so I'm cautiously optimistic that this particular issue should be solved. Hopefully, this will take care of things with the braking system as all the other brake components seem to be working well.

 

[cam_1100]

Well.....so much for optimism. Same exact problem as before. Any application of the rear brake pedal and the outer pistons seize up. If I open the rear bleeder valve, they release. Something, somewhere is blocking the flow of fluid. I'm really hating the complicated nature of this braking system at the moment.

Maybe I can just take off the rear rotor. I hardly use the rear brake on my other bikes anyway....

 

[cam_1100]

I assume that you are joking, right ?

Well....half-joking

I assumed when you said thst you cleaned the SMC in post #12, that you meant the cylinder bore ? You need to see whst is going on inside the SMC bore. Clean it out, ensure the piston can move freely. Be aware that the old fluid contained water, and the bore itself could be rough due to corrosion.

Yes, I cleaned the entire SMC and all the parts in it (spring, cap, piston, plunger). When I was done, it looked as close to new condition as I could have hoped. There was no debris remaining and the inside of the cylinder bore was smooth and clean. I meant to take more pictures, but by then it was late and my hands were covered in brake cleaner/brake fluid and I didn't feel like messing with my phone.

Check again that the problem remains with the SMC and hasn't moved elsewhere. It may clean up. It may need a new piston, it may need a new SMC.

Yeah, that's the plan. From what I checked before, I can't imagine the problem could be anywhere else. I may try one more time to clean it up and I'll spend some more time trying to isolate the problem. I'd hate to spend the money on a new one only to have this problem remain.

Thanks again for all the advice!

 

[cam_1100]

Updating the saga:

I reconfirmed that the rear outer pistons were locked and the problem seemed to be at the SMC by checking the following:

• Able to compress the center piston by hand, so the rear brake system is not stuck.
• "See-Saw" the outer pistons by hand, proving the pistons are not stuck
• Could not compress the outer pistons simultaneously, either by hand or with a clamp, meaning the secondary system is holding the brake at the rear.
• Opened the system on the input side of the SMC at the left-front caliper bleed valve and then the banjo bolt on the SMC, but the rear remained stuck, indicating a problem between the rear brake and the SMC.
• I opened the banjo bolt on the output side of the SMC and this released pressure on the rear outer pistons. This seems to indicate that the problem is within the SMC.
  • Incidentally, I reclosed the banjo bolt on the SMC and I could compress the outer pistons a small bit, but they bounced back. This indicated there was now air in the system, which would be expected from opening the banjo bolt with no pressure from the fluid supply.

No surprises there since I had done all of that before. So I tore apart the SMC again and re-inspected all of the parts. They all seemed clean and free of blockage as best as I could tell. Here are all the parts laid out in the order they are assembled and the best shot I could get into the cylinder bore, along with the SMC piston and rod. The surfaces are a little discolored, but free of debris. The rod gives a nice metallic 'clink' against the backing plate, indicating there is nothing preventing it from fully returning to open:



What I'm questioning now is the little green pod that goes into the input port on the SMC. I wasn't able to find anywhere that explained what it was or how it worked. As you can see from the image above, it goes into the SMC over the supply and compensation ports, then there is an O-ring and the cover plate. None of the parts diagrams of the SMC show this piece. It has screens on both sides. Inside is a valve made of a small ball bearing on the input side and a spring on the brake side. I took it apart the last time I cleaned the SMC to clean the filter screens:

Input Side


Output Side:


Internal Valve:


It seems that brake pressure from the input line will compress the ball-bearing against the spring and allow fluid to flow into the SMC. While it was still in the SMC, I blew compressed air into the input of the SMC starting at a very low pressure and increasing slowly. At first no air passed through, but when the pressure was high enough I could tell the valve opened and air began flowing through, so that seemed to at least verify that guess.

But the spring seems to close off any return path for fluid from the brake side. Based on what I was able to find online resarching brake systems I suspect this may be called a "residual valve", but I'm not sure. I'm confounded as to how the brakes on the ST work normally with this in place. It seems like pressure would never be allowed to release in the secondary circuit because this appears to only allow fluid to flow in one direction. It's possible that this is what is causing all of the problems I am seeing, but it also seems to be working as intended so I don't know what to think. I'm tempted to remove the ball and spring to see if that allows the rear brakes to work as I expect, but I'm obviously worried about altering the brake system like that for normal use.

Does anyone here know anything about this part and how it works??

 

[cam_1100]

Thanks John - I don't think I had found the article you linked in my searches yet, so I'll read through that next. Honestly though, I've read so many posts and articles from so many places it's getting hard to remember what I've seen and what I haven't.

Since I last posted, I did find This Thread (post #2) that mentions a part on an ST1300 similar to mine and the one you posted. It links to a diagram of the part and indicates there is a tiny return passage for fluid, similar to what you show in your diagram. I assumed there had to be something there, but I haven't been able to see it and my compressed air method is probably a bit too crude to detect it. I suppose I'll do my best to make sure this part is clean and reassemble. I wish that part was available on its own, but it's looking like a whole new SMC is in order if my efforts fail. I just hope there is one available. I tired to order a rebuild kit for the SMC from one site and was notified they are on indefinite back-order. I think I'll cancel and start looking for a new SMC.

Regarding the piston in my photo, I don't have a new one to compare but the "socket" is smooth enough to at least pass the fingertip test. I can't feel any pitting or protrusions on either the piston or the push rod. The push rod moves the piston freely and returns quickly and I can hear a "clink" when the back of the push rod returns all the way to the backing plate, so I fell like there are no obstructions there. I am about as confident as I can be that the mechanical parts of the SMC are working correctly given that there still seems to be a problem somewhere. I'm still suspecting a hydraulic flow problem, but I've been wrong before.

I was also reading that not getting the PCV bled correctly can cause the rear brake to bind. This seems counter-intuitive to me since air in the system usually tends to soften the brakes. I'll try to make sure I follow all of the guides you and others have posted before my next test just to be sure. I still don't think that is my problem because my brakes bind hard, lock the pushrod in the SMC, and make the rear pedal very stiff. The whole system seems to be locked up. I would think if there was air anywhere in the system, that would not be the case. At any rate, it's probably a good to get in the habit of bleeding everything properly....and I've definitely been getting a lot of practice.

 

[cam_1100]

But isn't it visible on your photo on the inner ring in the 12 o'clock position? You just need a very fine wire to push through - but you would want to be certain that it is clear.

Once again, you are absolutely correct! I don't know how I missed it, other than perhaps I thought it was just some leftover debris/discoloration. I was focusing mainly on the crescent-shaped 'well' behind the main port. After some additional cleaning, it is much more apparent.


I also removed the mesh on the input side of the valve, which I had been hesitant to do before because it didn't seem to be easily removed and I was worried about damaging it. With that screen off, I could easily see the relief port on the input side as well. It had been previously covered by gunk trapped in the mesh. This also allowed me to clean the mesh better, which it badly needed as you can see comparing the before and after:



As for clearing the relief port...that's another matter. The only wire I have that is fine enough to fit isn't stiff enough to force through. I can insert it 1-2mm then it either hits blockage or gets hung up in some other way and the wire just bends. I can't really get any force behind it to push it through. Given the condition of the fluid when I got the bike I suspect if there is a blockage it is crystallized(?) brake fluid. I'll look for a more suitable wire to clear the passage and, in the meantime, soak it in something to try to break up the old fluid deposits.

Is there an official name for this part? I would call it some kind of valve. Considering it isn't shown on any parts fiche I've seen and isn't mentioned in any repair manuals, I'm assuming it would be nearly impossible to order a replacement for this on its own (which is highly frustrating) but knowing what it's called might help.

Anyway, I have something to go on now. If I can get this port cleared I'll put everything back together and try the brakes once again.

 

[cam_1100]

Larry (@Igofar) once suggested a top E steel guitar string. That's what I've just done to test it out. It works a treat.

I have many high E strings ranging all the way down to the extra light gauge sets I use which are 0.010". I saw Larry's suggestion a while ago and considered it, but compared to what I was trying (which barely fit) I was thinking the guitar string would be much too thick. But if you say it works I'll definitely give it a shot.

Probably won't be able to get to it tonight (plans for wife's B-day), but I've got a nice open weekend coming up with spotty weather in the forecast, so I should have lots of time for working in the shop. With a little luck I'll have it sorted soon!

 

[cam_1100]

I talked to Larry (Igofar) yesterday on the White Courtesy Phone. He turned on his fire hose of knowledge and gave me a bunch of ideas for sorting out the brake system. I'll be following his suggestions, but in the meantime I wanted to follow through on the suspected blocked relief port in what I am calling the "input valve" (since I don't know the real name) on the SMC. I tried using a high-E string from an old guitar but could not get it to fit. So I went back and snipped the tail off the high-E from a guitar I knew was strung with an extra-light gauge set (0.010") and with a pair of small pliers* was able to force it through the input valve relief port. I was rewarded with about a 3mm long plug of dried brake fluid coming out of the opposite side:



The guitar string is the straight wire in the back, the wormy looking thing in the red circle is the plug of dried fluid. I'm hoping that might be what is at the root of this problem. But if something that miniscule can cause a problem like this, I think a thorough breakdown and cleaning of all the master cylinders is in order, as well as yet another flush of the brake lines. However, in the interest of furthering my knowledge on this problem, I'm going to put things back together and see if clearing that tiny plug solves my current issue.

*I had to use pliers to pinch the string and push it through. When I tried to use my fingers, the string kept going into my fingers instead of into the hole.

 

[cam_1100]

After bleeding the system, the locking brake problem seems to have been cured. When pressed, the rear pedal is firm and brakes hold tightly, when released I am able to easily spin the back tire. It seems we've tracked the root of this particular problem. Of course, now that the corrective maintenance on the brakes is complete, the preventative maintenance begins. I still have some work cleaning and flushing ahead of me, but it's a huge relief to have this fixed. I'd like to take it for a quick test ride to really check things over, but the weather is pretty awful today so that will have to wait. I'll check back in here with an update when I do.

I have to give enormous Thanks to John (jfheath) for walking me all the way through this and to Larry (Igofar) for sharing his wisdom. Your help was invaluable in getting it figured out and giving me hope that it could be done!

 

[fnmag]

After bleeding the system, the locking brake problem seems to have been cured. When pressed, the rear pedal is firm and brakes hold tightly, when released I am able to easily spin the back tire. It seems we've tracked the root of this particular problem. Of course, now that the corrective maintenance on the brakes is complete, the preventative maintenance begins. I still have some work cleaning and flushing ahead of me, but it's a huge relief to have this fixed. I'd like to take it for a quick test ride to really check things over, but the weather is pretty awful today so that will have to wait. I'll check back in here with an update when I do.

I have to give enormous Thanks to John (jfheath) for walking me all the way through this and to Larry (Igofar) for sharing his wisdom. Your help was invaluable in getting it figured out and giving me hope that it could be done!

Congrats on sorting out the brake problem.

 

[cam_1100]

Well...it seems I still have some work to do. The sticking brake problem is gone, but now it takes 2-3 pumps of the rear pedal to apply pressure to the rear caliper. I assume this means there is air in the lines somewhere. It also fails the manual SMC test which is another indicator. One other odd thing is that I can hear a noise coming from the SMC/Front left caliper area when pumping the rear brake until the pedal firms up. I'm thinking this means I might have air somewhere in the SMC, but I'm not sure where air could get trapped there without just moving along the line to the PCV and rear caliper.

And air in the SMC doesn't necessarily explain why the rear brake pedal isn't applying pressure to the center piston of the rear caliper, unless it's just a case of the fluid taking the path of least resistance to the air pocket in the SMC, but I need to familiarize myself with all of the interconnects from the rear MC and understand the delay valve's behavior a bit better to be able to say for certain.

I'm also searching the forum for the procedure on bleeding the PCV on an 1100 (which I believe DOES NOT have a bleed valve like the 1300 does), and to see if there are any other tricks I might have missed for getting all the air out. I'm sure I've read something here about a few steps not covered in the manual, but I've poured over so much information here while trying to solve my previous problem, I can't recall if that only applied to the 1300 or covered the 1100 as well.

Anyway, still plenty to do but at least I can move the bike around without having to take the rear caliper off now.

 

[cam_1100]

I bled the system again, this time I also opened the ports on the SMC and the input to the delay valve. It seemed to help to where the rear brakes are now solid on the 2nd pump instead of the 3rd or 4th, with the 1st pump having low-pressure instead of no-pressure. Next time I'll open the ports on the PCV as well in case any air is hanging out there. I'll keep chasing those air bubbles and hopefully get it right soon.

The good news is the front brakes stop the bike very quickly...and the ABS works!

 

[Igofar]

Left front caliper upper bleed valve, using the front brake lever.
Right front caliper upper bleed valve, using the front brake lever
Left front caliper lower bleed valve, using the rear brake pedal.
Right front caliper lower bleed valve, using the rear brake pedal.
Rear caliper front bleed valve, using the rear brake pedal.
Rear caliper rear bleed valve, using the rear brake pedal.

If you need me to call you tomorrow, I'm free most of the day.
Larry

 

[cam_1100]

If you need me to call you tomorrow, I'm free most of the day.
Larry

Oh yes, I'm quite familiar with the procedure. I've done it about 30 times in the last couple weeks and I've worn out both my kids making them work the rear pedal for me...

I've got stuff going on tonight and tomorrow, so it'll probably be a couple days until I can take a better look at it. I ordered one of the Motion Pro bleeders you recommended and it should be here by then. Hopefully that will simplify the process and help me do a better job of getting all the air out.