Originally Posted by
Winfried
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.
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).