C4 Power Booster Issue (AGAIN!!!)

'94 LT1
Awhile back I had a leaking booster that I had replaced, and everything was all good. Now I return from a month vacation and it appears the booster is not
functioning correctly due to a hard pedal action. The vacuum line is not leaking and the check valve appears to be working as well. Before I replaced the booster there was a hissing sound coming from the foot well, but all is quite now, which would suggest the diaphram is intact. I did apply a vacuum to the line to the booster and it seems it pulls air continuously from the booster.
Is this normal???
I thought that the vacuum would draw the sealed air from the booster and maintain a set vacuum. It seems to me that the shell may be leaking.
Can anyone suggest any other tests?
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