So something is happening more and more frequently with my sub. The brakes are pressurized and rock hard when I go to press them before starting up the car. It usually happens overnight. Once the car is cranked and the booster works, the pedal drops to normal pressure. Also, and I think this is ever so slight... but the brakes feel a bit spongy to me. Is there a booster check valve, would it cause this, and where is it located?