I have a 1994 F150 XLT 4X4 with 5.0 v8, I have replaced the brake booster several times (every month lately) the master cylinder is dry and not leaking into the booster, check valve is replaced with booster every time, New fuel pressure regulator, new brake lines, new brake pads, rotors, drums, shoes, and fluid. About 1 week after changing booster when I start truck I hear a hissing from under dash by brake pedal and when I push on the brakes the hissing goes away, also when I push on the brakes sometimes the pedal is VERY hard and I have to stand on it to stop. The pedal seems to be getting harder more frequently, what is causing this? Im on my 4th booster and im getting sick of replacing these every month, it is a lifetime waranty one from autozone but dont want to change it continueusly in my lifetime! Please help! There don't seem to be any other vacume leaks. Any help would be GREATLY appreciated thank you.
Call the manufacturer for help with this. They are going to autopsy the returned boosters to see why they failed. If there is a common cause related to your application, they will have a service bulletin or some recommendations. If there is a problem with their rebuilding procedures or the materials they use, they are going to figure that out so they don't have to keep on providing free replacements.