I installed the new reproduction dual tandem brake booster/MC/rod assembly in my '68 Coronet. The MC was bled while in the car using the manual method. Engine was started and I have no pedal. When shutting off the engine I hear hissing on the cabin side of the booster. After a few seconds when the hissing stops, presumably when pressure equalizes in the booster, the pedal is rock hard. I repeated this 3 times and noticed the same result.

The Ram Man has been helpful and responsive until I noticed this issue last weekend. I need to know if the booster needs to come out or if it's something minor like replacing the check valve.

With winter behind us it's killing me not being able to take it out for a drive.
Thanks