rear brake pistons very tight

[jimichu]

so while changing the rear brake pads and flushing brake fluids last week, I noticed one thing: the rear brake caliper pistons are very hard to push back. I cleaned up the pistons really well with brake fluid, and then I tried to use a clamp to push all three back together, it was very tight, I ended up push them back one by one with the clamp, it was easier. However, on the front brakes caliper, there's not much resistance at all, I can push back the pistons with my fingers (I accidently pushed the left front brakes pads together while the caliper is out and tilted, the middle piston went all way out). My bike has 65000 miles on the clock, and the rear pad is down to the change mark. I don't think the SMC is stuck but I really couldn't figure out why the rear pistons are so tight. After this morning's ride, I stopped and checked the brake rotors, the rear rotor is much hotter than the front rotor, although not smoking hot. (one factor could be the front end has much better air cooling)

anybody have any suggestions? or is it just the nature of this bike?

 

[TerryS]

Rear calipers get a tough life and see a lot more crud than the front ones, and they are less visible and easily rinsed out. You may have some debris caught between or under the dust seal which is increasing the friction on the piston, or you may have some other hydraulic reason (e.g. the SMC) that is preventing the pistons returning easily. You could test whether it is hydraulic by cracking the bleed screws before trying to push the pistons home (just wash the split fluid off any paintwork afterwards). If they move freely, then you know the resistance is hydraulic not seal friction.

If necessary, removing the caliper and rebuilding with new seals is not a big or expensive job, and check the pistons are free of scoring or corrosion before putting it back together.

 

[SMSW]

I just bled my brakes and changed all the pads. And yes, my rear pistons were harder to push in than the fronts - not impossible, but they definitely offered more resistance despite a thorough cleaning first. I did notice that if I did not immobilize the other pistons with a wood block, pushing one in results in the other one being pushed out.

 

[jfheath]

The rear centre piston has a direct line to the rear master cylinder, and if the rear master cylinder isn't clogged, then this one is relatively easy, only having to push the fluid through the tiny compensation port..

The two outer rear pistons however take a rather complex route - to the proportional control valve, then to the SMC - fluid is pushed through the tiny compensation port, then up to the delay valve and finally to the rear master cylinder where it is pushed through another tiny compensation port.

So yes - the two outer pistons are difficult to push in. I seem to make better progress if I see-saw the two outer pistons. Just check that the SMC is fully extended - pull it back by hand. It should return by itself but if it doesn't that will make the rear pistons harder / impossible to push. It also heralds the start of a different problem.

But before you start to worry about that - make sure that there is space in the rear reservoir for the fluid to return to. If like me, you flush, bleed and top up throughout the life of the brake pads, pushing the pistons back in will return more fluid than the reservoir can hold.

If it remains an issue, open up the bleed valve with a tube attached and push the fluid out that way. Some argue that you should do it this way anyway - as you may be pushing old fluid and gunge back into the brake lines - but if you bleed and flush yearly then I can't see this as being an issue.

[edit] I should have said that you MUST make sure that the exposed part of the pistons are thoroughly clean before pushing them back in. I use the hem of an old towel moistened with brake fluid and wrap it round each piston in turn and towel it like you would when drying your back. It is easier if you push the pistons out a bit first.

 

[Soontobeone]

The rear centre piston has a direct line to the rear master cylinder, and if the rear master cylinder isn't clogged, then this one is relatively easy, only having to push the fluid through the tiny compensation port..

The two outer rear pistons however take a rather complex route - to the proportional control valve, then to the SMC - fluid is pushed through the tiny compensation port, then up to the delay valve and finally to the rear master cylinder where it is pushed through another tiny compensation port.

So yes - the two outer pistons are difficult to push in. I seem to make better progress if I see-saw the two outer pistons. Just check that the SMC is fully extended - pull it back by hand. It should return by itself but if it doesn't that will make the rear pistons harder / impossible to push. It also heralds the start of a different problem.

But before you start to worry about that - make sure that there is space in the rear reservoir for the fluid to return to. If like me, you flush, bleed and top up throughout the life of the brake pads, pushing the pistons back in will return more fluid than the reservoir can hold.

If it remains an issue, open up the bleed valve with a tube attached and push the fluid out that way. Some argue that you should do it this way anyway - as you may be pushing old fluid and gunge back into the brake lines - but if you bleed and flush yearly then I can't see this as being an issue.

[edit] I should have said that you MUST make sure that the exposed part of the pistons are thoroughly clean before pushing them back in. I use the hem of an old towel moistened with brake fluid and wrap it round each piston in turn and towel it like you would when drying your back. It is easier if you push the pistons out a bit first.

 

[ST-Traveler]

Suspect SMC problem.
I replaced the SMC 2 years ago and the pistons push in as easy as the fronts.
Before replacing I had same issue as you are describing.

 

[STArnie13]

My 03 had the same problem and it was the sub master cylinder. Another 03 had the same problem at the same time. Seems like the 03's have that issue. Hopefully the later years don't.

 

[technotony007]

SMC

 

[Anna'sDad]

I noticed one thing: the rear brake caliper pistons are very hard to push back.

The next time you're working on the rear brake caliper, try testing the piston resistance by pushing the pistons back with the bleed screw open.

With the bleed screw open, the resistance of the hydraulic system components (tubing, hoses, various valves, and the master cylinder) is removed from the test. If the pistons still seem to have excessive resistance to movement, you can be confident that the resistance is with the piston, piston seals, and/or the caliper. Keep in mind that rubber brake components typically do not react well with petrol-chemicals, seals tend to swell up in size, which if forced back into service may offer significantly greater resistance to movement.

Additionally, with the bleed screw open (don't forget the bit of tubing to direct the flow into a container) you won't be pushing dirty brake fluid, and whatever crud may be in the caliper, back through the brake system components.

If you're interested in learning more about the brake system here is a post discussing the Dual Combined Brake System (DCBS).