My SRAM Rival road discs kept dragging despite the usual calliper reset thingy so I took the calliper off. One piston appeared to be working but not the other. I restrained the good piston and the other one started working good as gold. I repeated the operation a good few times, pushing the piston carefully back each time. However when I released the good piston everything went back to the beginning again, good piston worked, bad didnt move.
What can I do to get the dodgy piston working properly again? I have cleaned round it thoroughly, can I use any sort of lubricant? If it was Shimano I would just smear with the appropriate brake fluid and keep working it until both pistons worked equally.
I would just smear with the appropriate brake fluid and keep working it until both pistons worked equally
That's what I'd do but I also wouldn't expect them to ever be perfect. It only takes a tiny difference in piston-threshold-pressures to make one piston move before the other 8)
But pistons are retracted by the elasticity of the piston ring, rather by negative pressure in the system. Lubricating the piston against the rubber gasket would only make things worse.
I appreciate no two Pistons will move exactly the same but one is moving freely and the other not at all. So now one pad is worn out and the other is hardly touched and most annoyingly , dragging on the disc no matter how you move the caliper
As otsdr points out above, the piston does NOT move through the seals. This is the biggest misconception around. The piston is an extremely tight fit in the seals; it needs to be to prevent leakage of fluid. When you pressurise the system the seals deform allowing the piston to move half a millimetre, which brings the pad into contact with the rotor. Release the lever and the poston springs back to its earlier position. However as the pads wear the pistons do creep very slowly through the seals as the travel exceeds the limit of elasticity of the rubber.
The little spings don't push the pistons back; they are only there to help prevent the pads from rubbing the rotor when retracted.
The caliper needs to be centred exactly on the rotor and there needs to be no air bubbles behind one of the pistons causing unequal movement.
i think that there are three answers
1. disposable shimano
2. rebuildable hope
3. clean, bleed, new pads, recentre. repeat.
Lazy Sram brake pistons?
I used to have problems with lazy Sram brake pistons but then I swapped them out for Polish ones, they go all day now 100%. Cheaper as well.
otsdr - Member
But pistons are retracted by the elasticity of the piston ring, rather by negative pressure in the system. Lubricating the piston against the rubber gasket would only make things worse.
true but not sure from OP if piston is sticking out or sticking when retracted - suspect the latter as says
"However when I released the good piston everything went back to the beginning again, good piston worked, bad didnt move."
rightly or wrongly I've used just a bit of olive oil on the seal in this situation plus quite a bit of pumping out/pressing in with the working piston locked in place obviously cleaning off any surplus oil before refitting pads and centring caliper
Sacrificed far too much riding time on Avid Juicys with this...binned and got Shimano in the end
The annoying thing is the bad piston isn't actually stuck, when you lock the good piston it moves perfectly and pushes back as normal. It is just the inequality thing. Like several above I have only ever had one set of Avids, that was enough. So save for a Shimano set up seems to be the best answer!
By changing the name from AVID to SRAM didn`t solve the problems .....That they hoped it would 😉
Avid should have stuck with their cable brakes they were brilliant.