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XT single pot brakes. Have just started to stick on and bind pretty badly. Bled and fitted new pads today but the issue is persisting - pull the lever a couple times and the pads bind on and the lever sticks in a 'closed' position and won't ping back open.
Not experienced this before - need help from the STW hive mind. Is it just a case of lubing and 'exercising' the pistons or something more terminal?
the lever sticks in a ‘closed’ position and won’t ping back open
That's not the caliper, that's a problem at the lever end.
Does sound like the lever but if the pistons are also stuck, the Park Tools video below worked for me yesterday - string and IPA
https://www.pinkbike.com/news/tech-talks-fixing-sticky-pistons-park-tool--video.html
Had a Deore brake do the same, lever seals were knackered. The caliper is still going strong on the commuter bikes with a lever purloined from another broken brake.
How many times do I have to post this on here? Brake pistons do NOT slide in and out of the seals. If you've ever tried to remove a piston you'll know they are such a tight fit that it's simply not possible. The brake works when hydraulic pressure deforms the seals allowing the pistons to move closer to the rotor, then return when the seals revert to their original shape. There is no spring in the caliper to push the pistons back. As the pads wear, yes, the pistons do creep through the seals but that takes place over days or weeks.
Your brake is faulty somewhere up at the handlebar end.
“There is no spring in the caliper to push the pistons back.”
But isn’t there a spring in the master cylinder which pulls them back?
Sticky Pistons?
If they were Hope brakes.
If they were Formula.
If they were Magura
https://www.youtube.com/watch?v=JJI9ZrFMC1Y
There is no spring in the caliper to push the pistons back.
The seals function as a spring that retracts the pistons enough that they don't drag. In this case, it sounds like the problem is at the lever end, but sticky pistons can also cause problems because one piston will over-extend to compensate for the other one that's stuck. This causes the brakes to drag, even though the pistons don't slide back and forth each time you apply and release the brakes. Making sure the pistons are clean and not stuck is a sensible place to start troubleshooting. Cleaning and lubing the pistons before fitting new pads is a good way to prevent problems later.
Cleaning and lubing the pistons before fitting new pads is a good way to prevent problems later.
Thanks all.
Pistons were lubed but not 'exercised' when I put the new pads in.
Given it's Shimano, time to find some new levers then...
As a PS; I believe that what happens when a brake sticks is that the piston does jump though the seal and then there's nothing to pull it back so you get a dragging brake. It happens when you grab a big handful of brake and in my experience this happens on older brakes, possibly due to the rubber seals becoming less flexible and always when the pads are quite worn. Pushing the piston back in is a proper pain in the backside thanks to the tightness of the seals.
I think what happens is that one piston jams and the other extends normally. The rotor flexes towards the stuck piston and the other piston becomes over-extended. The over-extended piston resets itself in that position and the rotor then straightens up when you release the brakes, causing the rotor to drag.
FWIW, both pistons are extended normally and evenly to clamp the disc. They just won't retract when the lever is released, and the lever doesn't 'ping' back to its testing position as normal.
I had this with a factory bled XT brake. pulled the hose out of the master cylinder and there was a piece of debris stuck in the end of the hose essentially creating a 1-way valve. Brake locked up. Removing the debris and a mini bleed to remove the air bubble and all was good again.