I have a 1991 f350 super cab my problem is brakes the pedal is hard as a rock it only used to do it when i back up now it's all the time . I replaced the hose from the booster to tee on fiewall and from tee to pump on front of motor still same . Any ideas ?

 

Check your master cylinder and if your master cylinder is fine then it must be the vacuum pump! let me know what happens