91 F-250 4x4 7.3 IDI - Went to apply brakes and the pedal was harder than a rock. Panic city. Eventually got it stopped. I am thinking that the problem is probably with the power brake booster. Brakes would work off and on properly on the way back home. Luckily I was only about 3 miles away and all country roads. My question is how to properly diagnose this. To rule out the vacum pump on the engine or the brake booster. Any advice will be greatly appreciated. Thanks:dunno: