Hello scotty i have a 1991 f250 with 4.9L v6 engine. I took my truck out on a hot day up the mountain and on my way back down my brakes gave out. I had to e brake until i got to the base where it flattened out, since the day ihad it towed home i have had zero brake pressure, i have bleed all 4 brakes, front disc and back drums, i have installed 2 new calipers and a new master cylinder then re bleed everything including the rabs system, the problem i am having is when the engine is off i have brake pressure but when i turn the vehicle on my brake goes all the way to the floor. Without putting more money into it could you help me maybe solve my problem . thank you for any help you can provide.
replace the brake booster, it's bad