Brake issue assessment. Would you agree with my conclusion?

The setup:

A friend of mine calls me up and tells me that his rear caliper on his 5 Gen locked up on him. He asked if what I thought the issue was and I could check it out. My first thought was either the SMC or the main MC had a blocked return path or that the caliper pistons were stuck/corroded. He asked if I could help him with a rebuild and would it make sense to do a SS line replacement at the same time. "Yep, best time to do it" I told him. So that was the task.

So, after replacing the seals on all three calipers, then rebuilt the clutch MC, the front MC, the rear MC, the SMC and replacing all the OEM rubber lines with SS lines, all appeared to be going great. Bled the clutch, bled the fronts and then bled the rear. Clutch, works great, the fronts, work great, the rear...not so much! The same issue, the rear caliper stays locked. Note we did have a couple of pistons in the right front caliper that didn't want to come out but using a little compressed air and using a pair of spreader pliers, we were able to coax both of them out. Both were replaced for good measure.

The Issue:
Here are the knowns...
1. When pressing the rear pedal, the center pistons on both the left and right front calipers will engage.
2. When releasing the rear pedal, the center pistons on both the left and right front calipers will release.
3. When engaging the SMC (doing this by hand), the center piston of the rear caliper will engage.
4. When releasing the SMC, the center piston of the rear caliper will release.
5. When pressing the rear pedal, the two outer pistons of the rear caliper will engage.
6. When releasing the rear pedal, the two outer pistons of the rear caliper will NOT release.

Doing a little trouble shooting, when pressing the rear pedal and holding the caliper where we can see the pistons, only one wants to come out right away, the other barely moves. If we hold back the one that moves right away and press the pedal again, the one that barely moved, will move slightly and with further presses of the pedal will then move it out more. However, if we try to push the pistons back in, both are a little stubborn but the one that barely moves takes a lot more force.

The conclusion:

So, considering that the SMC engages and releases no problem, as well as the front center pistons engage and release no problem when using the rear pedal, I have to think that it is NOT the rear MC that is the issue but the either one or both rear main pistons are the culprit. Considering that the lines are new, the MC was just rebuilt, the caliper seals are new, plus the issue we had with the right front caliper and all other aspects of the braking system seems to work as intended, would you agree with my assessment?

Is there any chance the caliper is not perfectly parrellel with the disc or not perfectly vertical to the disc? Could the caliper hanger have been bent somehow? Is there any evidence that the pads have worn abnormally? 

Doing a little straw grasping here!

Excellent write up of your issue Duc2V4.

Good Luck. 

8 hours ago, Grum

Is there any chance the caliper is not perfectly parrellel with the disc or not perfectly vertical to the disc? Could the caliper hanger have been bent somehow? Is there any evidence that the pads have worn abnormally? 

Doing a little straw grasping here!

Excellent write up of your issue Duc2V4.

Good Luck. 

Without taking measurements on the caliper, I couldn’t say for sure but it looks to be sitting straight and looking at the old pads, other than being a bit glossy, didn’t seem to be unevenly worn. I suppose that it’s possible a caliper that was out of alignment could have deformed the pistons by putting pressure laterally on them. When I removed them to replace the seals, although they had a little build up on them, they didn’t appear to be deformed but again this was visually and not a measured assessment.

In all honesty, this one has me perplexed, all things really point to it being the pistons, even though I would have highly suspected it being the SMC/Proportional valve being the culprits or the MC return passage being blocked. Since the issue is not the center piston, the first two don’t even come into the picture, at least based on how the combined braking system works. Being that the fronts do not stay engaged when the pedal is released, I had to rule out the return hole being blocked on the rear MC. We took apart just about everything but the PCV and thoroughly cleaned and rebuilt everything so I’m really hoping replacing the rear pistons will resolve the issue.  

I don't know if this would be feasible/practical, but what about swapping out the entire rear caliper with a unit that is known to be working properly?

2 hours ago, Samuelx

I don't know if this would be feasible/practical, but what about swapping out the entire rear caliper with a unit that is known to be working properly?

Feasible but not really practical, as it would mean either taking one off of another bike, which means two brake bleeding processes. The other would be to find one not on a bike, online or from another member who knows that it was in good working order, installing it and hope that it works. The latter would mean buying or getting one on loan, then buying it if it ends up working. The owner would prefer to just replace the pistons at this time. If that doesn’t resolve the issue, we’ll fall back and regroup.

An interesting tidbit.. on the ST forum for ST1100s/ST1300s .. There is a lot of threads about the SMC and the rear brake locking up.. there have been crashes... It goes as crazy as an oil thread... It seems the same problem is inherent to the basic design across multiple platforms. 

48 minutes ago, mello dude

An interesting tidbit.. on the ST forum for ST1100s/ST1300s .. There is a lot of threads about the SMC and the rear brake locking up.. there have been crashes... It goes as crazy as an oil thread... It seems the same problem is inherent to the basic design across multiple platforms. 

I hear what you’re saying but the SMC is working like it should. It’s the main pistons that are getting stuck, not the center one, which is the one controlled by the SMC.

^It was just a general comment about the common design as a whole.... Sounds like on yours, just guessing..(my caveat... I'm very much a hands on, see it, smell it guy). you need to pull the rear and do some bench work to get the pistons to move equally...

You probably already did this?

I would be de mounting the rear caliper leaving the brake lines attached and working the pistons out with the rear brake pedal then forcing the pistons back in with clamps until all came good.

Was there no scale or corrosion on the rear two main pistons?

7 hours ago, Zarquon

You probably already did this?

I would be de mounting the rear caliper leaving the brake lines attached and working the pistons out with the rear brake pedal then forcing the pistons back in with clamps until all came good.

Was there no scale or corrosion on the rear two main pistons?

Yes we did do this and the pistons did have a bit of build up on them before we rebuilt the caliper with new seals but they were thoroughly cleaned before we put them back in. Other than the pistons and the PCV, everything was disassembled, cleaned and replaced with OEM rebuild kits.

I wonder if the rear pistons are sitting true in their pots after pushing on the back of the brake pad.

Perhaps rotating the pistons 90 degrees in their pots and/or replacing the rear pads if the wear is uneven.

In your original post I got the impression one of the pistons was a bit sticky. I'm wondering why? If the brake has been locking on then things would have been getting pretty darn hot. Something might have changed shape.

4 minutes ago, Zarquon

I wonder if the rear pistons are sitting true in their pots after pushing on the back of the brake pad.  In your original post I got the impression one of the pistons was sticky.

As far as I can tell the pistons are sitting in correctly and yes, before we tore everything apart, the pistons were a little dirty but we cleaned them up and they looked OK. Again, cannot tell for sure without measuring, so when the new pistons come in, we'll see.

Sorry I added some extra thoughts to my last post.

45 minutes ago, Zarquon

In your original post I got the impression one of the pistons was a bit sticky. I'm wondering why? If the brake has been locking on then things would have been getting pretty darn hot. Something might have changed shape.

This is what I’m thinking, if all other aspects of the braking system is working like it should, warped/distorted pistons is all I can think of. We have new pistons on the way, if this doesn’t resolve it, then I’m back to the drawing board. Pretty much everything is new or rebuilt at this point. 

Have your friend do a quick visual inspection of the brake lever assembly.  Just to make sure that Part #13 is installed in the correct orientation.

(If it's installed backwards it can affect pressure bleed-off back into the master cylinder reservoir when the lever is released)  

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5 hours ago, GreginDenver

Have your friend do a quick visual inspection of the brake lever assembly.  Just to make sure that Part #13 is installed in the correct orientation.

(If it's installed backwards it can affect pressure bleed-off back into the master cylinder reservoir when the lever is released)  

Hey Greg. How would this have any effect on the two outer pistons of the rear caliper, aren't the rear two outer pistons hydraulically isolated from the front system?

Update for you. So going around the block to get next door, turned out to be the SMC after all. However, the two rear pistons after being replaced (as well as the center one) started working like they were supposed to. What we didn't seem to notice the last time is that, although the SMC was activating the center piston like it was supposed to, it did it from a relative piston position.

To clarify that last statement, if the center piston was all the way in the caliper, the SMC would pop it out its typical length but would release. If the piston was partially out, it would again pop out its typical length and return back in. However, if the piston was out far enough to press the pads to the rotor, the center piston would not release. So in a sense, the SMC was working like it was supposed to when activated but not in a standing position. This is something we over looked when we had the two outer pistons give us fits.

At first I thought I could get away with tearing the SMC apart completely and resolve whatever was causing the issue with a thorough cleaning. Alas, that was not the case. As luck would have it though, SFDownhill had an SMC laying around from his 5 gen brake De-link, so I was able to grab the one he had and installed it this morning with the assistance and moral support of fellow member HammerDrill. I pulled the old one off and put the used one on and bled the system (it goes real fast with speed bleeders BTW) so within a manner of a few minutes i was able to test the system and viola! it worked. All I can say is that if there was anything plugging up the SMC, I couldn't see it. All I can think of is that there's a little check valve inside the SMC where the main and return holes are and it was a bit worse for the wear cleanliness wise, but I did pull it apart and give it a good cleaning.

SMC with check valve in place.

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Check valve removed and both holes clean and unobstructed. The light you see is from my flashlight I placed at the mouth of the piston hole.

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The check valve as viewed from the top.

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The underside of the check valve. If you open this little goodie up, you'll find a ball bearing held in place with a spring that is on the under side of this cap. Not sure if this goodie is beyond its service life or maybe I really didn't clean it well enough. I will say there was a "light" mud substance in the SMC and the owner of the bike said he had some of the same substance in the caliper, so who knows.

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Thanks for the detail.  I like the linked brakes and hope to keep my system clean enough to avoid any issues but it's good to learn about the innards.

Changing brake and clutch fluid is key IMO.  On all cars and bikes.

Duc, did you also notice in that little blue valve that there is another of those teeny tiny compensation ports built in? Fluid pressure from the foot brake pushes the ball valve open to fill the SMC but any pressure coming back from the piston has to pass through both the compensation port in the metal of the SMC and the port in the plastic cartridge. I'd suggest you had some crud caught in there. 

18 minutes ago, Terry

Duc, did you also notice in that little blue valve that there is another of those teeny tiny compensation ports built in? Fluid pressure from the foot brake pushes the ball valve open to fill the SMC but any pressure coming back from the piston has to pass through both the compensation port in the metal of the SMC and the port in the plastic cartridge. I'd suggest you had some crud caught in there. 

Yes I did and had to be something still corrupting the SMC, as once I put the one that SFDownhill gave me and refilled the lines, worked like it was supposed to!