Having hydraulic disc brake pads too close to each other leaves no place for the rotor to spin freely. There are several different reasons why pistons don’t retract completely. It’s actually a widespread problem, and some causes are straightforward to fix. However, there are deeper mechanical problems that can cause pistons to be out of position as well. These sometimes require an experienced mechanic, but you can repair them by yourself most of the time with proper guidance.
How to fix hydraulic disc brake pads being too close
Hydraulic disc brake pads need to be pushed apart so that pistons retract into their default position. Pistons are pushed back by inserting a clean flathead screwdriver or similarly shaped object between the brake pads and forcing them apart.
Sometimes pistons won’t stay in default position but rather move back out as soon as you stop pushing them or get stuck halfway there. This indicates several possibilities and needs to be explained thoroughly.
Hydraulic Disc Brake Pads Are Too Close After Taking the Wheel Off
This has, most likely, happened to everyone. I know it did to me multiple times because I was a bit careless. Taking the wheel off to change a tube or something, and once you try to put it back, you can’t. The brake pads are too close, and the rotor won’t go between them.
Somehow, the brake lever got squeezed, and since there was no rotor to block the pads, they stuck together. Luckily, it’s rare to see them completely stick together so that you would require something very thin to separate them. In most cases, there would be enough gap to use a tire lever to push them back. That way, even if this happens somewhere during a ride, you can still fix it.
Resetting Hydraulic Disc Brake Pistons After Changing Brake Pads
While brake pads wear out, brake pistons protrude out to account for the wear. That way, the brake lever always has the same travel from the default position to the “bite” point. Once it is time to change the pads, you should push the pistons back completely before inserting new brake pads.
This is very important because it will make it a lot easier to center the brake caliper and adjust it, so both brake pads are equally distanced from the rotor. More importantly, when you grab the brake lever, both brake pads will touch the rotor at the same time and apply the best possible braking power. Believe it or not, doing this wrong can reduce braking power by a lot.
Hydraulic Disc Brake Piston Is Not Retracting
I believe this is one of the most annoying things because it can happen on any brake if the brake fluid is not perfectly clean. The hole between the brake system and the reservoir in the brake handle is very tiny, and dirt can shut it easily. If I haven’t worked on the same brake previously, I’m not sure if someone bled the brake while pads were still inside the caliper, and pistons weren’t fully reset, which is another reason why this happens. So, to troubleshoot this problem, you need to know how the brake was serviced previously or test the brake system for both causes.
I like to start with checking if someone improperly bled the brake. Although there are times when this type of bleeding is actually helpful, it causes problems like these in the end. If it were, there would be an excess of oil in the system, preventing pistons from retracting completely. In most cases, it’s an easy fix by removing the oil cap on the handle and pushing pistons to their default position. If there was any excess oil inside, it should drip out. Just make sure to put something beneath so it doesn’t make a mess.
If this doesn’t solve the problem, that means the reservoir hole is closed by dirt. Don’t let this surprise you; the small dirt particles come from metal, plastic, and rubber from which the brake is made of. All brake fluids are corrosive and will damage anything over time. In this case, it’s best to remove all the brake fluid, disassemble the brake handle and clean it. You will see the tiny hole in the oil reservoir, which you can clean with a thin needle. Although this isn’t difficult to do, it requires experience and precision. Some brake handles need to be disassembled completely, while others just need a top cap removed to access the whole oil reservoir.
I would make sure to google how your brake looks inside before you try to do this yourself. Some brake handles are quite difficult to put back together. After everything is clean and assembled back, you need to put new brake fluid in the system and properly bleed the brake.
Hydraulic Disc Brake Piston Is Stuck
A piston can get completely stuck as well. This usually happens with older brakes that aren’t ridden for some time. Since I have multiple bikes with hydraulic disc brakes and really little time to ride, some of them sit in my garage for months at a time. I’ve seen this happen twice on those bikes, and it was always after not using the brakes for a long time.
I’m guessing pistons corrode, and if they are not moving, the oil from the brake system can’t lubricate them from the inside. Ideally, I would take apart the caliper, check the seals and everything, lubricate the pistons with brake fluid, and put everything back together. However, the process of it is a bit difficult to explain.
Luckily, there is a trick that works just fine. I take a bleed syringe with just a bit of brake fluid and squeeze it through the brake caliper onto the stuck piston. Just a tiny drop is more than enough. You don’t want to make a mess that will contaminate brake pads that need to be taken out to do this safely. Then I just force the piston inside to unstuck it, and the brake fluid should do the rest. However, if you can’t push it inside the caliper, try doing it the other way around. Insert something flat into the caliper so that the other piston gets blocked before the stuck one. Then just squeeze the brake lever really hard until the stuck piston pops out. After that, you can lubricate it the same way I mentioned above before pushing it back in.
Why You Should Retract Pistons From Time to Time
Resetting pistons should be included in the hydraulic disc brake maintenance from time to time. In my opinion, this is the best way to keep a hydraulic brake working for ten or more years. This is important because brake fluid corrosion doesn’t do as much damage when parts are moving regularly as it would otherwise. Even if you use your bike every day, pistons never fully retract unless the brake pads are brand new. The fact that pistons have restricted movement, and a part of them protrudes out not being lubricated by the brake fluid tends to build up resistance in their movement.
I’ve done this on the rear brake on a bike I have, while I never did it on the front brake. Both brakes are the same Shimano XT model. The front one started having problems with the right piston, while the rear one never had any issues. Recently, I disassembled the caliper and found out that both pistons had difficulty moving. It’s just that the right one was getting stuck first, so I noticed it. I disassembled the rear caliper to do a complete case study, and the situation was completely different. It seems pretty clear to me that retracting pistons occasionally can help keep them working perfectly for many years. Since it requires 2 minutes of my time, I decided to do it regularly on all of my bikes.
I was surprised to find that no one mentioned anything related to this anywhere. A good pair of hydraulic disc brakes would be equally great and valuable 10 years from now if maintained properly. The performance didn’t change much with new hydraulic brake models; it just shifts from one brand/model to the other.
They are not a bike part to neglect in terms of maintenance.