I am about to replace the brake booster on my Ford F150 but am not sure it is bad. During cruising, everything is quiet. When the brakes are applied, I then hear a loud hissing in the cab. If I do a few panic type stops, the pedal gets slightly harder to press each time. After stopping the engine and pumping the brakes, there is little power assist available. I was told by an "expert", that it may be some gasket or seal associated with the booster because it does not hiss all the time. I would appreciate any suggestions.