My son's 1993 LX 5.0 Mustang has a braking problem. The front brakes lock up during any strong braking. It is to the point that it is dangerous! We go to track days at road courses, so locking up the front brakes is bad! We recently bought the car. The PO used it for autocross. The front brakes are off of a 99 to 04 GT, with a 5 lug conversion front and rear.. The rear are the stock drum brakes. Since we have gotten the car, we have replaced almost everything on the brakes. We replaced the front calipers in case they were sticking. We replaced all the brake lines with stainless steel lines, in case they were failing internally and replaced the master cylinder. Since we carry it to the track, I wanted to replace those items anyway. They still lock up with minimal effort. It is dangerous on the street. I now believe it is due to the larger front disc brakes. The brake line routing is strange, with three lines off of the master cylinder. One to the driver side front brake and the other two to the proportioning valve. Two lines leave the proportioning valve, one to the passenger side front brake and the other to the rear. I need advice! Do I put two adjustable proportioning valves on each front line and adjust each? Add disc brakes to the rear to get better brake bias. Has any one else had this problem? Any suggestions? I'm not letting my son drive the car at this point and as an 18 year old, he isn't happy with Dad!! HELP!