I had the same problem on my 96 f250 couldnt find out what was wrong. I even changed the master twice thinking i got a bad one. but i did end up talking to another mechanic and found that theres a Rabbes valve or how ever you spell it that would make this happen its located on the driver frame rail like under the door area maybe little before. Cost me like 150 new but I get it for cost. It has something to do with the ABS . after i changed it never had a problem sense real easy to install.