Car is a 1990 740GL with ABS.
Symptoms: When the brakes are applied there is a distinct air hissing/rushing sound which stops as soon as the brake pedal is released. When the engine is turned off with the brake applied, the pedal almost immediately starts pushing back against my foot and goes almost rock-hard in 4-5 seconds.
The car also exhibits an occasional high idle (1000+ rpm).

I checked the vacuum hose from the manifold to the booster and there are no cracks or splits. I removed the check valve and can blow air from the booster side through to the hose side but not back the other way. I assume this is correct since I compared it to my 940 which was the same. The check valve itself is not cracked or otherwise damaged.

Is it possible to remove and dismantle the booster to check for a broken diaphragm? Is there any other way to be certain this is a faulty booster? Is the check valve known to leak around the rubber gasket on the booster?

I would really appreciate any help people can offer.