slow lever return- Slow leak or sticky piston?
Background- I have several sets of older shimano m755 4-pot brakes... I've had great luck with these brakes over the years, love the performance and I have tons of small parts for them, so I'm not really eager to replace.
One set has been upgraded to straitline levers, hope hoses and stainless hardware.
On this set, I've had consistent problems over the last year with the rear brake. The main symptom is slow/incomplete mushy lever return. The first few times, it also seemed as if the brakes were losing fluid- sure enough, after a dry 5k' descent I found signs of very slight leakage where the hose goes into the caliper (just a tiny bit of oily dust). I replaced all hardware at this area with goodridge and it's been leak free since.
I still encounter the mushy, slow lever return though, particularly when the bike has been sitting for a while. A fresh bleed always seems to fix it, perhaps just cause it's oiling up the seals and pistons? The pistons do not seem sticky- all 4 move equally. I am bleeding from the bottom up using a syringe and typically cycle a good bit of extra mineral oil through the brakes so I know I'm getting a good bleed.
The brakes are quiet and dry so I don't see any leakage.
It really seems like the issue is with whatever spring forces the lever to return- but I can't see this spring. Is it inside the MC? I have an extra rear lever with a busted clamp, so I could try swapping the MC guts- anybody tried this?
If I need to replace, I'd prefer to just replace the levers or the one caliper, but how do I isolate the problem?
Any ideas? I'm not excited to replace these but bleeding every 8-10 weeks is getting old.
Last edited by FM; 11-02-2008 at 01:16 PM.
I remove my XT calipers, and then the pads, and I close the pistons gently one by one with a set of pliers, then open them with the brake lever then close them etc.. until they are all working throughout their range and most especially returning to the open position.
I don't need to do this often, maybe once a year, but I think the pistons returning fluid through the line is what makes the lever return.
Open, close, close, open, you get the picture.
Incorrect, a spring on the MC piston is what forces the lever to return, otherwise it would never do it just based on the motion of the pistons and their miniscule return force due to the seals.
Originally Posted by Prophet Julio
Yeah, i really don't think sticky pistons are at fault. I've watched the pistons while squeezing the lever with the wheel and pads removed, all 4 pistons move equally.
Originally Posted by Jerk_Chicken
The more I think about it, the more I suspect the MC. I have a spare lever with all the replacement parts, but no idea if an MC rebuild is possible. Any thoughts?
BTW- I looked for an exploded view of the MC on shimanos website. This is as close as they provide, not very helpful.
edit: what does part #9 actually do? I can't imagine that it provides any return. The bill of materials calls it the "boss spring"
I'll probably try to pull my old busted lever apart and see what's inside. Hopefully it's straitforward.
Last edited by FM; 11-02-2008 at 05:07 PM.
The diagram appears to miss out entirely the actuation piston, seals and spring. The assembly would look something like the two components at the bottom left of this picture...
I would be very surprised if it was the spring that turns out to be at fault, more likely, in my opinion, of being one or both of the actuation piston seals.
What use is a philosopher who doesn't hurt anybody's feelings? -
They almost all work the same. This is an Avid, can't find a Shimano one.
Last edited by yetibetty; 03-23-2009 at 11:24 AM.
Hey thanks guys!
I got it sorted- turns out the problem was with part #10 in shimanos diagram below (nothing to do with the MC!). The only way I figured this out was by tearing the MC apart in my old busted lever and comparing all the parts to those in the sticky lever. Turns out part #10 was out of tolerance (or perhaps it had warped?)- anyway for whatever reason it was sticking in the lever blade and preventing the boss from rotating to allow the lever to fully return. So I just replaced it with the piece from the loaner parts lever and now everything appears to be good.
Here's the guts of the shimano lever- nearly identical to the hope!