Overfilled master cyliner = Blown?

[BaliLover]

I did the rear brakes on my 240 yesterday night, and fixed a leaking rear caliper by replacing the seal around the piston. I replaced pads and rotors and bled the line going to the caliper I fixed. My father was the one adding the brake fluid and watching the level while my mom pumped the brake while I bled the caliper. Somehow my father overfilled the MC and put the fluid right to the top of the resevoir and put the cap back on. Driving home last night the brakes felt much better than before, more firm for sure. Today I go for a drive and at first the pedal felt good, then it got mushy again. I pull over and pop the hood to check the fluid, and see its RIGHT at the top of the resevoir and appears to be leaking from the cap. I clean the MC off and go inside to get something to remove some fluid from the resivoir with. By the time I got back it was leaking again, but this time it looked more like where the resevoir meets the MC.

Is it possible that the combination of having fixed my leaky caliper and regaining proper brake pressure, and overfilling the MC caused it to blow out the seals?

[SmithSR]

Very unlikely. 99% of seal failure is workmanship. I'm not saying there's something there that isn't causing a failure, but years of experience brought me to that high number. A car's worst enemy is the DIYer that forgets a step or instruction(myself included, we all learn from our mistakes)

If the M/C is overfull, remove some fluid or re-bleed the system and try again.

[BaliLover]

I didn't rebuild the master cylinder, didn't touch it except to remove the cap so fluid could be added as it was bled. The only thing I touched was the rear brakes, and the only real mechanical part was replacing the piston seal, which installed fine and is not leaking.

[SmithSR]

Oh wait, the M/C is leaking. I could never read anyways...Yeah, how old is the M/C?Many fail just as you describe.. my original did.Usually old age, wear and tear does it. in between the reservoir and the cast housing is where they leak and destroy all sheetmetal.

Replace immediately!

napa, schucks, autozone all should have em!

[BaliLover]

Yeah, that was my next plan of attack. I've already replaced the one on my coupe which had less mile than this hatch so it was only a matter of time I guess. It was probably the slow leaking rear caliper that kept it alive this long since it couldn't generate total pressure.

[cdlong]

you can't over fill the MC, aside from putting so much in that it pours over the top. the cap should have a vent in it so it's probably fine. like you already said, time for a new MC.