PWR brake pedal slow to return

[Kevin Mac]

47 chevy with 383, mild cam, the brake pedal is pretty slow to return up. If I come to a roundabout and just slow a little the brakes drag until the pedal comes up. It also does the same from a stop light. I know that low vac. will cause only having a good pedal once or twice but will it also cause the pedal to return slow and brakes drag? It looks to be a common booster MC combo frame mounted. I didn't install it so not sure what brand. I added a vac. cannister to help with more vac volume but made little to no difference. Before you ask how much vac. I can't remember when I checked it last year what it was. I am thinking around 12 inches if my memory serves me right.
Going out to shop and work on truck, will check back in a bit.

Thanks Mac