I read several topics in this forum but none really defined a problem I am seeing with my brake system. I am running a 7" single diaphram, 1" bore Corvette style M/C which is under the floor. With the motor off, the pedal feels good, not spungy or low. When I start the car and step on the pedal it drops several inches and becomes rock hard. The pedal always retracts but I have nothing to about 75% pedal throw then its hard. All the components in the system are new but that doesn't say much. I am pulling about 15 psi vaccum at idle and I am thinking the something in the booster might be bad, the check valve is bad or a return spring would help.

Does this sound like a bad booster problem?