Non MB Brake system problem - PeachParts Mercedes-Benz Forum

BigMikeT57 · #1 06-12-2004, 11:41 PM

The car is a '88 929 with 146/k.

Problem, brake pedal goes to the floor with little brake effect.
Found master cylinder empty. Refilled with DOT3, pumped pedal a few times and pedal pressure diminished quickly. Fluid level low again. No external leaks seen.

Refilled the resevior and watched as the fluid level dropped without the engine running or touching the brake pedal.

Some general research on the web suggests its the master cylinder leaking into the power booster. I'm getting conflicting solutions, replace the MC, replace the booster, replace BOTH.

Thanks in advance.

Mike

sbourg · #2 06-14-2004, 12:43 AM

There should be no way for brake fluid to get out of the master cylinder into the booster, so if that is happening, your MC is bad. Disconnect the MC from the booster to check.

Steve

BigMikeT57 · #3 06-17-2004, 08:33 PM

Steve,

Thanks for the reply, I'll pull the MC this weekend.

Meza · #4 06-18-2004, 12:45 AM

Yes the master cylinder can leak into your booster and it's very common. As a matter of fact, booster that have traces of brake fluid in them can not be taken back for a core charge. If your booster is vacuum opearted, the leaking brake fluid into the booster will be sucked into your intake and the car will smoke heavy white fumes. It happened to me twice and will happen again if I continue to buy JUNK products from Autozone!!!!!!

Mark DiSilvestro · #5 06-18-2004, 10:03 AM

Since we're posting non-MB brake problems I have a weird one. A friend has a '97 Volvo 850 wagon with ABS. Her brakes developed a somewhat mushy brake pedal but will 'pump up' more solid. Not scary but definitely not the firm pedal feel I expect from an expensive European car. No visible leaks or loss of brake fluid. Bleeding the rears restores a solid pedal for awhile but after a few weeks or months the brakes are mushy again.
Any opinions or suggestions?

Thanks, Mark

Kestas · #6 06-18-2004, 10:21 AM

Mark, perhaps there's a problem with a piston seal at one of the calipers. If the seal is less than perfect, say, due to corrosion, the caliper can slurp air past the seal during brake pedal release. This was a problem with some early Corvette Sting Rays that used a lip seal rather than a full face seal. The lip can act as a one way valve for contaminants.

If you don't want to take apart each caliper, bleed the brakes and see which one releases air to narrow down the culprit. But if all calipers are original, it may pay to rebuild each one after seven year's service in the harsh northern climate.

Mark DiSilvestro · #7 06-18-2004, 06:26 PM

Thanks for the quick reply. I suspected something like a faulty caliper seal but had never heard of air getting IN through a bad seal without fluid leaking out. Glad to hear that this is a more than just a theory and has happened to others.

Happy Motoring, Mark