500E Leaking brake fluid when M/C put under pressure

[michaedo]

Hello good people,
long time lurker, occasional inputer.
Set about to bleed the brakes this morning. Schweben bleeder full of Pentosin DOT4.
I wasn't aware of having to bleed out the ABS unit. Anyway, got the pressure unit up
to 15 lbs. and fluid started coming out around the master cylinder. Before realizing
the leak I had attempted to bleed the right rear. No real fluid movement but did get
about 4oz. eventually.
So...am I looking to be buying a new master cylinder? it's the original from 1992.
I kinda know the answer, just looking for a knowledgeable affirmation.
Thanks in advance,
Don.

 

[gsxr]

Don, most likely the rubber grommets between the plastic reservoir and the metal master cylinder are leaking. Double check if that's the source of the leak. If so, just replace the grommets. Part number 000-431-15-50, two required. Now if the leak is from the REAR of the master cylinder only, by the metal vacuum booster, that's a different story. (I really doubt this, but it can happen.)

Pulling 4 ounces (120cc) from each caliper is fine for a typical brake fluid replacement. I usually pull a bit more from the rears, a bit less from the front. Make sure you follow the FSM procedure at this link (see page 2) including bleeding from the SP port on the ASR/ABS unit.

Usually you want about 30psi pressure but if the process works ok at 15psi, that's fine. Typically only 1 quart/liter of fluid is needed.

 

[michaedo]

upon closer inspection the master cylinder is clean and dry. The fluid looks to be coming
from the bottom of the booster. Once the pressure was relieved the leaking quit. Looking
at M/C for sale there is quite a price variation. In your diagram, sealing ring, #11, popped.
That's my bet. Thanks for your prompt reply. Don.

 

[gsxr]

Don, you'll need to do more investigation first. If the leak is from the booster, the booster could be full of brake fluid (but, you would have been adding a bunch of brake fluid over months/years to make that happen). If the booster is full of fluid, it will also need replacement in addition to the MC. The booster should always be 100% dry inside. Leaks out the rear of the MC are pretty rare.

Seal ring #11 is an air seal between the MC and booster, which is under vacuum with the engine running. Replacing that won't help. You will need to unbolt the MC from the booster and check for liquid out the rear of the MC and inside the booster.

My money is on a leak from the 2 reservoir grommets, dripping down to the bottom of the booster...