Jump to content

Need pointers for cleaning SMC (secondary master cylinder)


V4Ever
 Share

Recommended Posts

I am trying to resolve the problem of rear brake dragging.  I have narrowed the problem down to the middle piston not releasing.  This would seem to point to a problem with the SMC.  I just reviewed an old post titled "Rear Brake Locking After Pressing Pedal" posted by iThinkergoiMac which seems to describe my issue exactly.  

So I pulled the front left brake braket/SMC off the bike.  The service manual I downloaded from this site (thank you!) describes the disassembly of the SMC master cylinder as basically "remove the bracket/SMC from the bike, remove the rubber boot, and remove the snap ring.  I'm stuck at 'remove the boot'.  How does the boot get removed?  I assume the snap ring is hidden under the boot and not accessible until it is removed.  It doesn't appear it can be slid off with the mounting bracket/saddle shaped thing still attached.  Any pointers would be much appreciated.   I was able to remove the green screen/check valve and blew out the crud, so that part should be OK.

 

IMG_3935.jpg

Link to comment
Share on other sites

I have no idea what condition the piston is actually in, since I can't figure out how to remove it. When installed on the bike, I can move the SMC as it would normally move under braking.  It did not seem to actually apply any pressure to the rear center piston.  Applying only the foot brake would cause the center piston ( actually all 3 psistons) to apply pressure, and would not release unless I opened the center bleed nipple on the brake. 

In the earlier post I referenced, Vee-Ef-Ar posted a comment "When the foot lever is actuated, the secondary master located on the front left caliper is back-pressurized so that the braking of the front wheel does not cause additional braking at the rear. In short, using the linked foot lever for braking delinks the other system."  

This would seem to indicate that using the foot brake only, would cause all three pistons on the rear brake to apply pressure.  This is what I am experiencing, and the center piston does not release. 

Link to comment
Share on other sites

Just reread your comment, I was thinking you referred to the piston in the master cylinder.  I have removed and cleaned all of the pistons in the rear brake, and installed all new seals.  All pistons can be moved with finger pressure

 

  • Thanks 1
Link to comment
Share on other sites

  • Member Contributer

If you push the boot down you will see a 10mm lock nut that holds the clevis in place. I put a spanner on that to hold it and then used a large adjustable wrench to rotate the clevis, which should unscrew off the threaded end of the pushrod. There are some that will tell you the length of the clevis/piston assembly is critical, so it would pay to measure the distance from the clevis to the SMC housing before you start so you can put it back to the same place. (Personally given the movement of the SMC/ piston, I don't believe it is quite that critical)

 

After that you can pull the rubber boot out, and that will expose the circlip holding the piston. 

 

If the centre piston is not retracting then you most likely have a blockage in the tiny compensating port in either the SMC body or in the plastic check valve which you have cleaned. Hopefully you spotted the tiny hole in that one. The compensating port(s) is the only way for the line to depressurise and allow the piston to retract.

 

Moving the SMC should activate the rear piston; if not you may have some air in the system. Pushing the foot pedal will also pressurise the back centre piston (pressure goes around the tapered seal of the  SMC piston) and the two outers pistons.

 

When I worked on my brakes it took me a few tries to properly bleed the SMC and PCV. My wife helped me to press the brake pedal to pressurise the SMC, then press the SMC and release the PCV bleed, close the bleed, then press the pedal to push the SMC piston back out. And repeat. You do need to have the SMC angled so air rises to the blind end. 

Link to comment
Share on other sites

  • Member Contributer

Until you pull the SMC fully apart, the compensation ports are currently like Schrodinger's cat, being both blocked and unblocked. 

Link to comment
Share on other sites

  • Member Contributer

image.thumb.png.24f41f2bb01512c6435b23689b835dc7.png

The circled spot is where the compensation port in the check valve lives. The check valve can be pulled right out, and then disassembled completely/carefully as it just snaps together. Don't lose the little ball and spring from the check valve! This photo is from my ST1300 but it has a very similar linked brake system. 

Link to comment
Share on other sites

Thanks Terry.  I did notice the nut below the clevis, and put a wrench on it and attempted to turn it while holding the clevis.  It didn't want to turn and I was unsure if I should force it until I was certain that was the correct method to remove the boot.  I had read other references to the importance of the length of the assembly.  

I also saw the tiny compensation port in the check valve.  I didn't have anything of the correct diameter, human hair is not stiff enough, so I blew it out with compressed air after disassembling the check valve.  It did have some sludge built up on the filter screen.  

I'll make another attempt on the clevis tomorrow.

Link to comment
Share on other sites

  • Member Contributer

Put some penetrating oil on the clevis and let it sit overnight. I've done this a couple of times and mine weren't that tight.

 

Supposedly you can pull the boot back far enough without removing the clevis to get at the circlip, but it didn't work for me.

 

I fired some brake cleaner through the tiny port in the check valve and made sure I could see a clear jet flowing through. 

Link to comment
Share on other sites

Well, one step forward followed by brick wall.  I was able to remove the clevis, boot and snap ring.  I think I have now found the root issue.  The piston is stuck in the compressed position, and will not budge.  I tried compressed air through the banjo bolt hole, finger over the other brake line fitting, no movement at all.  What are my options?  Considered soak in penetrating oil, would that ruin the piston seals?  Maybe if I reconnect the brake lines and pump the brake the hydraulic pressure might force it out?  Worst case I could plug the banjo bolt as someone else has done.  Any other ideas?

Link to comment
Share on other sites

  • Member Contributer

I think you should consider the seals as damaged in any case so I wouldn't worry about damaging them with penetrating oil. Hydraulic pressure would be the safest way to eject the piston, I've read of folks using a grease gun to do this but have never had to resort to that myself. Have you tried tapping the piston in further to break any corrosion?

 

You might need to think about getting a replacement SMC, as it is possible the bore is now damaged as well. 

 

 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy.