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Morning folks,
I’ve been puzzled for some time now and I think I’m getting closer to the root cause! Ive got some Sram Force hydraulic brakes on my CX bike. The rear brake is fine and feels really positive but the front has loads of lever throw before it bites.
On closer inspection, it looks like the pistons are fully retracting when I let go of the brake whereas the front pistons stay stuck out ever so slightly. I guess this should happen to compensate for pad wear? But the front doesn’t seem to be doing so.
Any ideas? I’ve bled the system many times and at best it’s always been pretty poor.
Cheers
sticky pistons. the bane of bicycle disc brakes. Pads out, pump the pistons out, lube with a bit of the correct fluid, work 'em in and out a few times being careful not to push them too far out, clean 'em up and reassemble.
When you pull a brake on the pistons move out and the seals distort. release it and the seals return to their usual shape pulling the pistons back. As pads wear then pistons should slide a little thru the seals after than initial distortion so compensating for wear. if the piston / seal interface is dry this does not happen leading to the symptoms you describe. using a ziptie to hold the brake on overnight might reset it. Its nothing to do with bleeding
tjagain - seconded 😀
Awesome thank you! Will give it a shot
sram/avid brakes in particular are prone to it more than others 🙁 shame cause for the first 3 months when they are new they work awesome - they just never seem to have that same spark again.
it looks like the pistons are fully retracting when I let go of the brake
It could be that the piston seals are sticking, but this suggests that there isn't enough fluid in the system.
glenh - fluid levels do not affect this unless you overfill to stop the pistons being pulled back into place by the seals. disc brakes are open systems - they compensate for fluid levels as the pads wear by letting air in above a diaphram
Yeah, could also be lack of fluid in the system. Do what TJ suggests and if you find that you can't get the pistons to advance even with the pads and disc out then it's probably lack of fluid.
The diaphragm is an elastic element that will tend to weakly encourage the pistons to retract. However, my expectation is that piston stickiness is at the heart of problems like this so I agree with all the above.
I've started experimenting with ultrasonic cleaning assembled brake calipers as the "lubricate with fluid" tends to attract further crud into the small clearances. You have to be sensitive to the fluid medium for the cleaning being compatible with seals and (in the case of DOT) the hydrophilic nature of the fluid means water based cleaning is probably a bad plan. I'm primarily playing around with problem Shimano calipers so non-DOT and they're backup brakes (having finally replaced them with Formula Curas) so it is a no lose scenario to see if I can bring them back to life.
As above, the problem isn't fluid level, it's sticky pistons.
peaslaker - the diaphragm will do nothing to pull the pistons back. Its the distortion in the seals that retract the pistons. Nothing else.
If the fluid level is so low that it won't push the pistons out then the brake will not work