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Old 04-02-2006, 09:14 AM
krs krs is offline
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Testing a Brake Booster.....Revisit.

I've read most everything on topic but need a refinement.

I have an 1979 123 MB brake booster that WILL pump up and hold pressure, but it takes a heck of a lot of MightVac pumps - maybe as many as 100 to get it to 15 inches (HG?).

Is it a good one or not?

(The car has NO power boost and the vacuum pump delivers 18 inches at idle, rising to over 20 at about 2000 rpm. The line is good)

I've looked at the 0-ring and found it looking good and feeling rubbery. No damp in the booster that I can see, and the M/C is good. Brakes are good but are not power brakes just now.

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Old 04-02-2006, 09:43 AM
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There is a lot of air inside the booster so 100 pumps on the Mityvac would be typical, IMHO.

If the booster is holding vacuum, it should provide the power assist that appears to be lacking.

Are you sure there are no leaks from the pump to the booster?
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Old 04-02-2006, 11:29 AM
krs krs is offline
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Well, I'm pretty sure that there aren't leaks but I'll try it some more. When I didn't get any suction on the only 'T' from the booster line I assumed it to be closed.
There's been some creative bypassing done on the car, looks like, so I need to verify the hosing against a diagram.

I had just thought it might be too much volume in the booster or that it might bypass internally causing it to need so much pumping to bring to a test pressure.

Shouldn't the vacuum out of the pump be higher than 18 at idle? I had understood vacuum to be at it's highest at low engine speeds. Or is that a gasser hangover idea?


Hmmm, I'm finding possible relevance in http://www.peachparts.com/shopforum/showthread.php?t=148910&page=2

Last edited by krs; 04-02-2006 at 11:35 AM.
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Old 04-02-2006, 11:49 AM
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You vac pump should be pulling 20"-25" of Hg (Hg is the element symbol for mercury). I'm think that you may have a leaky/tired pump. You may be able to get away with a new vac pump check valve. But a rebuild kit for the pump might be in order as well. Before diving into that though make sure that when you test the vac pump that there is nothing else connected to the main booster line, that way you are sure that you are checking only the pump and nothing else.

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