Hey guys, I seem to be having the same problem -- I feel that my brake booster is the source of a vacuum leak. I have narrowed it down by removing the main vacuum line from the booster and plugging it -- then I get up to 20 on the mity vac, and all works great. When I replace the main line onto the booster, the vacuum reading on any of the fittings leading off the main vacuum line drops to just over 10. Is there a further test I can do to find out where the leak is, or if there is one? I understood that I could have a bad seal where the master cylinder meets the booster, but I don't seem to have any fluid leaks. The fitting where the vacuum line meets the booster seemed very clean when I removed it, so I doubt any fluid is being sucked into the lines. Thanks a lot!

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1980 300SD (daily driver), 1981 300D (Inca Red)

Put the Mityvac on the booster. It should hold vacuum.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

How do you fit the mityvac to the booster in a reliable way?

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1980 300SD (daily driver), 1981 300D (Inca Red)

The seal between the master cylinder and booster face seals air, not fluid.

Remove the hard plastic vacuum line from the vac pump and block it with your finger while the Mityvac is connected to the T of the hard line (everthing else disconnected and plugged)

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

There's an o-ring between them that provides the seal.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

"There's an o-ring between them that provides the seal." Yes and that seal should be checked; an O-ring is cheaper than a Brake Booster.

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84 300D, 82 Volvo 244Gl Diesel

I just dealt with this in my 85 300D. I put a rebuilt MC on the booster while both were off the car, so bench-tested the diaphragm. I pushed in the pedal rod against the floor, then sealed the vacuum port with my finger. When I released, the rod extended like it would with a torn diaphragm, but I heard a hiss between the booster and MC. The MC was from Autozone ($45) and had a thin square O-ring. I replaced it with a thicker round O-ring from my Harbor Freight kit and no more leak.

I hadn't even thought to check the seal because all other boosters I have dealt with from 60's Chryslers don't have a vacuum seal between booster and MC. Indeed, there is a drain there for any leaking brake fluid. They have an internal bellows seal around the output rod. It seems the Girling design allows a thinner booster and indeed those on my 96 & 02 minivans look similar. Now I need to look at my son's 84 300D since I replaced that MC with an Autozone one a few years ago and he complained of sometimes "hard to press" brakes and sometimes has to use the "stop" lever to shutoff the engine (though I replaced stop valve). In both boosters I found a pool of brake fluid, which is why I changed the MC's. I repainted one. Be careful handling because the shell is aluminum.

Then that would mean that it's not an o-ring. Which is precisely why I referred to it as a "seal" and not an "o-ring."

Hey, I am not sure if it is an o-ring with a round section or square section. All I know is that I have replaced one in a brake booster with a round section o-ring and that fixed the vacuum leak. And in BillGrissom's case, a round o-ring also fixed the leak of the square o-ring. Maybe we can conclude a round o-ring works better in this case?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If the cross section is square, it's not an o-ring.

Technically, that is correct. I have an o-ring that's been under a vacuum pump in a VW for 30 years, when I took it out, it is square, no longer round. So that o-ring became a seal after 30 years?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Do deals bark or is it just pooches ?

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."