Hydraulic disc brake piston problem

I am in the process of replacing the brake pads on my SRAM Red 22 HRD
brakes. I have replaced the front pads with no problem, but when I try
to push in the pistons on the rear one they always return to (almost)
the original gap. Pushing in a piston on one side causes the other to
move back out, and vice-versa. I wondered if it was an overfilled system
on the rear brake but have never had cause to bleed or otherwise tamper
with the hydraulics.

On Tuesday, August 6, 2019 at 8:35:07 AM UTC-7, Kevin Edwards wrote:
I am in the process of replacing the brake pads on my SRAM Red 22 HRD
brakes. I have replaced the front pads with no problem, but when I try
to push in the pistons on the rear one they always return to (almost)
the original gap. Pushing in a piston on one side causes the other to
move back out, and vice-versa. I wondered if it was an overfilled system
on the rear brake but have never had cause to bleed or otherwise tamper
with the hydraulics.

It sounds like it is over-filled or you are not pushing hard enough on the pistons -- or the pistons have been extended beyond their seals and gotten stuck.

I'll defer to the DOT fluid experts on whether it can expand over time, but I would just bleed the system and see whether that fixes it.

-- Jay Beattie.