Rear Master Cylinder PISTON STUCK : SOLVED

[TimTyler]

I was helping my nephew bleed the brakes on his K75 a few weeks ago. I depressed the brake pedal too far and the piston got stuck in the master cylinder. The piston tip was flush with the edge of the MC. I tried unsuccessfully to pry it out with a sharp edge.  :dunno2:

I checked Clymers and they suggest taking it to a dealer who likely has a special BMW tool.

My nephew came up with a brilliant solution.  He cut the end off a bicycle pump's hose and clamped that hose to the MC's input from the brake fluid reservoir. He plugged the MC's output end. He applied pressure on the bicycle pump and, voilą, the piston popped out to its extended position. Problem solved.

Sorry - no pictures from his garage, but here's something to illustrate the solution.

[alabrew]

When I had a similar issue on the '91 RS. I found a nail that fit in the treaded end and hit it with a hammer to release the plunger.


My problem was a little different. I was replacing a leaky plunger and the new one kept get stuck in the cylinder as I bleed it. I tried to hone the cylinder with the brass cleaning brush of my gun cleaning kit. I tried everything. After multiple disassemble/assemble to figure out why it was sticking, I (finally) discovered that the metal washer backing the rubber seal was stuck to the plunger and would not allow the fluid to return.

[stokester]

Just like using compressed air to remove caliper pistons.  Just be sure to use care.