Ok guys, this is the gist of it: I have a 75' F250 Highboy that I put a 79' Dana 60 Kingpin front axle under. Bleed the brake system and took her down the road. I had brakes for about twenty mins, then the pedal, slowly, started to makes it's way to the floor. Parked the truck and a few days later, the brake pedal seemed to be firm. I got a rebuilt master cylinder from the parts store. After I put it on, I bleed the master cylinder, hooked the brake lines up and proceeded to bleed the lines, starting from the pass. rear/driver rear/pass. front/driver front. The pedal was nice and firm. As SOON AS I started the truck, the brake pedal went straight to the floor, no brakes. I rebled everything. The same problem. Can anyone tell me what I am doing wrong or what might be wrong with my system????? Thanks in advance. Shane Oh yeah, I am not seeing any fluid coming from anywhere, no leaks that I know of.

 

Booster??????

 

Have you tried bleeding them with the truck running? If your not seeing fluid leaking anywhere it would tell me that the master might be bad but you said you have changed that. Are you able to put a vacuum pump on the booster? Will the booster hold vacuum? If not it is probably bad.