Brakes rubbing on ride after centering, brand new bike

Question

I recently purchased a new mountain bike. The brakes are Clark M2 hydraulic disc brakes. The bike came pre-assembled except for the pedals, handlebars, and front wheel. I don't see anything wrong with the caliper/rotor alignment but this is my first mountain bike in 10 years and I'm not much of a mechanic.

The back wheel is centered wonderfully and hasn't had an issue, even when testing the brakes vigorously before a ride. It stays centered and the pistons seem to retract well. There was no rubbing before my first ride.

On the other hand, the front brake seems to habitually fall out of alignment. This is even when the bike is mounted. I'll center it, pump the brakes a few times, and after a few pumps the brake is rubbing again. I've tried my best, and recently thought I had it solved. I manually centered the caliper multiple times very carefully and played with the brakes. Except for a little bend in the rotor, it seemed good. The calipers are also fully tightened.

I took it out for a spin today, and to my dismay when I got back the entire left side is rubbing against the pads.

Like I said, this is my first mountain bike in a while and my first time dealing with hydraulic disc brakes. The bike and all the parts are brand new, so hopefully there are no defects.

I haven't tried bleeding the brakes and a local mechanic told me that lubing the pistons might do more harm than good. I'm just looking for a second opinion on what I can do.

Edit: Forgot to mention when I was installing the front wheel, I accidentally fully pressed the front brake. The pistons didn't quite meet in the middle and there was no fluid leaking. Not sure if this affects anything.

Answer 1

It could be a lazy piston, although these happen mostly for bikes that have actually been used, as opposed to bikes that are new. The most likely case is that you have a bike with two brakes, you use the front brake, and one of the rear brake pistons becomes lazy after riding the bike often in dirty conditions due to non-use of the rear brake.

How to find out if it is: with the wheel attached, center the brake. After centering it carefully, observe if it's still rubbing -- don't remove the wheel at any point. If it's rubbing, press the brake lever and observe carefully if the two pistons are moving the same amount. With a lazy piston, one of the pistons is closer to the rotor, moves less to reach the rotor, and when you release the lever, moves less back to its original position that's closer to the rotor.

A rubbing brake could also be due to a bent rotor so carefully check if the rotor is straight. In most cases, rotors can't be 100% true, but if the pistons are not lazy the rotors should still spin freely after centering (which might be required on QR bikes every time after removing the wheel).

If your rotor is only rubbing in part of its rotational range, it could be a combination of lazy piston with a very slightly out-of-true rotor. In this case, fix the piston: rotors can't be 100% trued with any reasonable effort. There will be very slight out-of-trueness anyway.

Lazy pistons plague hydraulic disc brakes. Although it may sound useful to have a brake that doesn't require turning a barrel adjuster occasionally (rim brakes) or turning two barrel adjusters very often (most mechanical disc brakes), you WILL have problems with the rear brake pistons on hydraulic disc brakes -- and with the front brake pads wearing at 5x-10x the rate rim brake pads would wear.

A fix to a lazy piston is to remove the wheel, remove the brake pads, press on the lever few times to extend the pistons a bit (don't press too many times or the pistons fall out), clean and lubricate the piston seals with a cotton swab wet of brake fluid, then press the lazy piston fully in with a soft tool like the rubber handle of a cone wrench, then do the same for the non-lazy piston and continue doing so, while pumping the brake lever to move the lazy piston nearly fully out (not so much that it falls out). Repeat pushing both pistons in and pumping the lever while holding the non-lazy piston few times, now if everything is fine the pistons should move the same amount for a while, until the rear brake piston becomes lazy again due to non-use in dirty conditions.