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  #1  
Old 09-29-2012, 09:20 PM
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Help with possible vacuum leak in brake booster

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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  #2  
Old 09-29-2012, 09:30 PM
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Put the Mityvac on the booster. It should hold vacuum.
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  #3  
Old 09-29-2012, 09:33 PM
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How do you fit the mityvac to the booster in a reliable way?
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  #4  
Old 09-29-2012, 09:36 PM
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Quote:
Originally Posted by sshanky View Post
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 seal between the master cylinder and booster face seals air, not fluid.
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  #5  
Old 09-29-2012, 09:44 PM
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Quote:
Originally Posted by sshanky View Post
How do you fit the mityvac to the booster in a reliable way?
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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  #6  
Old 09-29-2012, 09:46 PM
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Quote:
Originally Posted by qwerty View Post
The seal between the master cylinder and booster face seals air, not fluid.
There's an o-ring between them that provides the seal.
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  #7  
Old 09-30-2012, 12:02 AM
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"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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  #8  
Old 09-30-2012, 02:56 AM
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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.
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  #9  
Old 09-30-2012, 11:11 AM
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Originally Posted by BillGrissom View Post
...and had a thin square O-ring.
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."
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  #10  
Old 09-30-2012, 11:48 AM
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Originally Posted by qwerty View Post
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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  #11  
Old 09-30-2012, 12:39 PM
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Originally Posted by funola View Post
Hey, I am not sure if it is an o-ring with a round section or square section.
If the cross section is square, it's not an o-ring.
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  #12  
Old 09-30-2012, 12:59 PM
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Quote:
Originally Posted by qwerty View Post
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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  #13  
Old 10-01-2012, 09:43 PM
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Do deals bark or is it just pooches ?
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