andrew2

I have a bike with Shimano BR-M447 hydraulic disc brakes. I am having trouble with the brakes dragging so I am trying to understand what is wrong, one step at a time.

I have aligned the pads, pretty much following the technique given here: https://youtu.be/Vet9d_qwIik

For the rear brake everything works fine and everything spins freely with no drag and no noise (I suspect the front has a slightly warped rotor which I am yet to get to). The problem is that after leaving the bike for a few hours (on the repair stand, untouched) the pads are dragging again on one side. I can centre them again but again, after a few hours, the problem is back.

I assume there must be something being compressed when I do the alignment and slowly releasing that tension over time but I'm not sure what. When tightening the bolts I do very small turns alternatively on the bolts. Could it be that I'm not doing them tight enough? I couldn't find a recommend torque value in the manual.

My only other theory is that the hydraulic fluid is in some way compressing from me squeezing the brake lever while aligning and then expanding again - but if that was the case I would guess that more brake lever squeezing would make the problem go away but it has no effect.

This is a follow on from https://www.bikeforums.net/bicycle-me...ders-even.html

Thanks!

FBinNY 

My immediate suspicion is that you have some air in the system.

gsa103

QR front wheel? Tighten the heck out of the QR. If the QR isn't properly tight, the wheel can shift slightly under braking, leading to what you're seeing.

andrew2

It is QR but this is starting to rub without the bike leaving the work stand so no hard braking has occurred :-(

andrew2

Is there a way to test? I would have thought (but am just guessing) that in that case repeated squeezing of the brake lever should compress it again but it doesn't seem to...

FBinNY 

Hydraulic fluid is non-compressible, and so the shoe is "directly" attached to the lever and they move together. Air is very compressible, and air in a brake system acts like a spring would be if it were a cable system. So the air reduces the working shoe travel, by compressing when brakes are applied, and expanding when released.

Air also can expand or contract more than oil does with temperature changes so the shoe clearance may change between when indoors and out.

There's no simple test, but someone used to how brakes are supposed to feel will detect the added sponginess when squeezing the lever.

Otherwise, the only way to know is to bleed as if there is air. Before doing so, arrange the bike so the bleed port or connection is at the very top of the system. Then work the lever a few times, alternating with tapping on the lines (flick with a fingernail like nurses do with IV lines) to work any air bubbles to the top. Then do a quick bleed and see if you squeeze any more air out. If/when the weeping oil is clear you're OK.