Daleford

We took a chance on a used brake booster that was working well on a car that was freshly scrapped. Unfortunately once installed it was leaking back into the passenger compartment.
Can this, or any future used boosters be easily bench tested?
BTW this was a 94 plastic model booster.

CorvetteMike2024

I would give a good visual inspection on plastic body for cracks.

Corvette Mike

theadmiral94

In therory, if you were to install the check valve into the brake booster (or if it's already installed), hooking up a mity-vac (or equivalent) to it via a hose, and pull a 15-20 hg of vacume, should at least tell if the plastic housing is sound.

Read the FSM before trying this for warnings (seem to remember something), but you might then be able to depress the shaft (which connects to the brake pedal) to verify no real loss of vacume (the depression should open up the back side of the diaphram to outside air, while closing off the vacume to the back side of the diaphram at the same time, sucking in the shaft, and not allowing any vacume to escape to back side of the brake booster.



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Demonic85

What if the pedal is hard (like a bad booster) on startup and functions normally when the car warms up? Bad check valve or bad booster?

theadmiral94

Could be the brake booster's check-valve, However, more likely it's something else. All the check-valve does is hold in higher vacume when lower vacume is present. Easy test is after car has been run, pull of the hose (don't remove the check valve), and depress brake pedal a few times, if pedal seems normal 1-2 depresses, check valve likely is ok (can also listen for any leakage or use a small piece of paper over hose barb to see if it holds the paper there, implying some leakage).

Could be the booster shell (which when heated-up, seals a leak), but should hear a leak when cold and presume you've listened and not heard it (start, run for only a minute or so, turn off and listen).

Have you checked the vacume level at cold idle? Should be at least 15-20 hg.

If not, could be a broken vacume line, bad vacume 'check valve' (for AC controls), old PCV valve, intake manifold leak, throttle-body leak or butter-fly carbonned-up, or brake-booster's vacume hose routed to the wrong connector.

Demonic85

The heating up and sealing sounds like the most likely cause. I know all the connections are correct and no leaks in the manifold or hoses. The PCV was replaced not too long ago and its not gunked up. I couldnt hear any hissing because of how loud the engine/exhaust is. Guess I will have to get a stethoscope. If I do hear a hiss when the brake is depressed does this indicate a bad check valve?