Disk-brake Pistons?
 

Took my bike in to get the disk-brake pads replaced and they told me that the 'pistons had locked' and the brakes needed to be replaced, but would likely be covered by warranty.

Could anyone tell me if this is a common occurrence w/ disk brakes? Is it even a thing? Will it be covered by warranty? I'd just like to know if I'm being told the truth.

I don't know if you're being told the truth, but it is possible for the pistons to get stuck. I haven't seen it happen, even once, on a bicycle, so I would suggest that it's not common. I, also, can't imagine it would happen on both the front, and rear, at the same time, unless there was a manufacturing defect which should be covered under the warranty, unless someone added the wrong brake fluid to the reservoir and gummed up the works.

Thanks, Kevin. Yes, front and back. The wheels won't spin freely now, so the bike is not fun to ride. I had to hit to brakes hard to avoid a car a couple of days before I brought it in (I didn't stop as soon as I would have liked and thought the pads must've been worn).

I'll have to look into the warranty I suppose.

While I'm not the disk guy at work we have seem frozen pistons. It is uncommon for complete capture of the piston. But we see far more often build up of grime, pad residue, whatever getting imbedded between the piston, the cylinder and the gasket/return 'spring" and the movement gets stuck. While this can be cleaned out and parts replaced it's usually more cost/time effective to replace the calipers in total. Or that's what I've been led to believe by my co worker. Andy.

+1

Over here, among the all-year riders - particularly the commuters, the phrase used is to "exercise" the pistons.

Remove wheel and pads, a quick spray with WD-40 or similar product, wipe/scrape/dig out as much grime as a 30 sec effort will remove. Spray again.
Notice runoff color and content. Repeat spray-and-wipe if it seems prudent. Push pistons all the way in. Insert rotor substitute. Operate brake. If sluggish to return, repeat depress-and-operate sequence until good operation returns or patience fades.
Wipe off residual cleaning agent, install pads and wheel and call it good.

Do note that a suitable rotor substitute should be used to avoid pushing the pistons all the way out of the calipers. But it's faster to have something almost whatever flat and thick enough at hand instead of having to reinsert rotor and pads 2-3 times over.

Cheers, Andy. Bugger.

I'll add to this thread since the original question looks similar.

My bike has Shimano BR-M395 disk brakes that have been working fine for over five years now.

The front brake still works OK, but since removing the rear wheel the other day and putting it back without touching the brakes at all, I noticed it braked fine while on the stand but had close to no braking power on the road.

After removing the wheel + disk pads today and pressing the brake lever slightly, it turns out the right piston didn't move at all while the left piston did stick out, which would explain the issue.

What's the technical reason? Is the brake simply too old and I should get a new caliper?

Thank you.

https://cimg3.ibsrv.net/gimg/bikefor...17566956c9.jpg

You need to clean that up and exercise the pistons, clean again, exercise some more...until both pistons move uniformly. The way to exercise them is to hold one side and squeeze the brake levers so the piston that is sticking gets moved in and out. You will need something flat and sturdy...something like a dinner knife that you can hold the piston inside the caliper. You will need to push the piston back in a few times, each time cleaning it before doing so. I use mineral oil so nothing gets back into the system and contaminates the fluid.

So in your picture, you want to clean and push in and hold the piston on the left (as you are looking at the picture).

I had Hayes Stroker Ryde brakes as my first ever set of disc calipers. They were a wee bit finicky to set up without rubbing but otherwise worked well.

I used them for some slushy and salty winter riding then put the bike away until spring, not thinking about what the salt would do to the brakes. By spring the brakes were seized up and I couldn't get them unseized. Replaced with Shimnao SLX and the knowledge that they may need to be serviced if I ride in the winter.

Do you mean I should apply mineral oil — the same I use for the braking system — onto the right-side piston to help it get unstuck while I hold the left-side piston with a bake piston press?

Clean it first, then apply some mineral oil (yes same for braking system) around the piston before you push it in. You will need someone to assist you while you hold the piston in while the brakes are applied to push out the one that is stuck. Once you get the one stuck out apply mineral oil around that piston, push it in, then exercise it by holding the other piston in again while someone manipulates the brakes.

Thanks for the infos. I might try to hold the left-side piston with a clamp or an adjustable spanner, as an easier alternative to a brake piston press.