I have a 1993 Ford F350 with 304,000 miles. The brake pedal was feeling soft so I took it into the local garage. They checked the pad and shoes and said they were fine but that the booster was bad (said if you took the vacuum line off the booster the pedal was firm, but when connected it would go to the floor with constant pressure). Tried 2 different booster still the same problem, then changed out the master cylinder, again no luck. The mechanic has plugged the lines to the front and the system seems fine, but the problem returns when he plugs the lines to the back so he believes that it has to be in the front brakes. No leaks to be found anywhere. I am tired of buying parts that weren't the problem. Any ideas?