Brake booster leaking

[Ely]

Hi everyone I was hoping someone could help me out with this issue. So I recently installed a new brake booster as well as a master cylinder on my 1984 Mercedes 300d. However when I start the car I can hear it hiss especially after turning it off, the brake eventually get hard and I can't turn off the car. I can't seem to figure out what I am doing wrong when it comes to installation. This is the 3rd brake booster I have bought because I thought the first two I put on were defective. If anyone can chime in and tell me what it could be? I even put a new o ring between the cylinder and brake booster yet still leaking.


I know the vacuum pump works because when I plug off the line and use my mityvac it reads over 20hg. Any ideas

[barry12345]

It sounds like the pushrod going into the booster might be too long. There may be an eccentric adjustment on the brake pedal arm.

[Junkman]

You can use a MitiVac connected to the vacuum line to find out whether the MC leaks. Rig a way to close the big line and pull vacuum via one of the smaller lines.

[Turbo300Mercede]

I thought there was a sealing gasket that went between the booster and the firewall. Seem to remember that on an 80 model 300TD. That gasket may be your leak?
Please let us know what you find as the leak.

[vwnate1]

It's an O-Ring and is *very* important .

[BillGrissom]

I took the booster off my 1984 (or 1985, forget). I recall only a foam gasket "dust seal" between booster and firewall, as on most cars. As o.p. said, there is an O-ring between MC and booster, which seals vacuum. That seems common on boosters which are thin, pancake type, like the Girling in my 300D's. In my 1960's Chryslers, there is an internal rubber bellow around the booster output rod, so no need for a vacuum seal at the MC. Indeed there is an opening to let any brake fluid which leaks past the piston drip out (instead of go into booster as in 300D).

Many boosters have an adjustable tip on the output rod. You adjust is so it just barely pushes the piston in when bolting up. I don't recall if my M-B Girling booster had that. Many people seem to think the brake pedal should bottom out against the brake switch when released. The switch should only sense the brake pedal and not prevent it from moving where it wants to rest. Insure your switch isn't limiting pedal motion. If so, it might be like "riding the brakes", which would actuate the booster. I don't recall ever looking for that switch, so don't know if adjustable like in my old Chryslers (slotted hole on clamp). It isn't adjustable in my minivans.

[t walgamuth]

Wouldn't brake fluid in the booster be coming from a leak in the MC?