I have a 96 F-250 Powerstroke that I am having brake problems with. I replaced the sending unit on the master cyl that was leaking but the pedal has gotten hard like there is no vacum to the booster. With the engine running there is vacum but the light on the dash is on. When I rev it up the light goes out and I have a softer pedal. I can't hardly drive through town. This truck is meticulously maintained. What can it be?

You might want to check your vac. pump. When it begins to fail it can cause some brake booster problems.

vac. pump

Thanks Guys. My pockets are $140 lighter but I have brakes. I suspected the pump but that seemed too simple. Thats the way things work with me.