It's not an A body but I was hoping you guys could help. I am having troubles with my truck. The running gear is from a 1977 W250 and the body is a 1986 D150. The brakes have not been right since I bought it this summer. I replaced both front calipers because they were sticking. I bled them properly and still have problems. When I pump the brakes without the engine running the pedal feels good and barely goes down. Start the engine and the pedal goes to the floor and brakes are poor. I tried changing the master cylinder with another used one I had and the same problem. I checked the booster according to the manual and everything checks out ok. No vaccuum leaks. No fluid leaks. The rear brakes are adjusted properly. I have ran 2 quarts thru the lines to make sure I bled them good and still the pedal goes to the floor when I start the engine. The brakes grab enough to stop but not enough to stop in an emergency. I have to push the pedal as hard as I can to get it to stop quickly. I am talking about bracing up against the back of the seat and giving it all I got. Has anyone had a similar problem? I am thinking it is the master cylinder but wanted to see if someone else had the same issue before I order parts. Any help would be appreciated.