I am working on a 1968 camaro w/power brakes. I've just recently replaced all the rubber brakes hoses front and back, new pads/calipers and shoes/wheel cylinders. I bled the system and the brakes work but not very well at all. When I hit the brakes there's a hissing sound coming from the booster and I have a hard pedal but weak brakes. It seems like my rpm's climb when I hit the brakes also. Anyone have any idea if this is a bad booster or just a check valve? The master cylinder and the booster are about the only things I haven't replaced, but I cleaned the master cylinder pretty well and flushed it out.