Langadorf

A mechanic checked out my brake system for free, and said my mushy pedal was being caused by a leaking brake booster. I hooked up a vacuum hose to the booster and sure enough, when I pushed the pedal I could almost freely suck air through the hose. So I went and got a booster from the parts store I did this same test on this one and the booster seemed airtight. I went to install it and noticed a messed up thread so I exchanged it. So the next one I did this same test on, and it behaves just like my original one did. I can suck air freely through the hose when I depress the plunger. This made me wonder if this was a valid way to test these things. Is there an official/proper way to test the air-tightness of a brake booster?

Big Fish

Looking at a sectional view of the booster, the only way you could get a vacuum leak to atmosphere is a bad diapragm or a faulty floating control valve. The GM manual really doesn't go into diagnosis of the booster as much as I would have expected, but it definately sounds like you have a problem with yours. They do indicate the unit can be rebuilt and parts can be replaced rather than replacing the entire unit. For an internal leak the recommended fix is dis-assembly and replace defective parts as described in the overhaul manual (which I don't have). All the internal parts are accessed by splitting the shell and having at it. Hope this helps?