After the car has set for a couple hours, with engine off, pump your brakes. You should be able to get a few power assisted pedals before the pedal gets hard. If you get a hard pedal from the start, either your brake booster is leaking, the check valve or grommet is bad. If you didn't know, the check valve is that 90 degree plastic piece which is plugged into the brake booster.I have even had the grommet crack which the check valve inserts into. Any of these can cause your problem. Rarely but it does happen, power assist can be marginal while driving, some cars have a vacuum storage can or ball which is supposed to store vacuum during hard acceleration,they can a spring a leak.