I bought a 93 mustang with front brakes upgraded to those of a 99. whenever i press the brakes with any force the front brakes lock up and slide. im not slamming the brakes. this has been a major issue as i track the car and its getting dangerous on the street as well. we tried replacing the pads, calipers, lines, and master cylinder. my uncle, a mechanic, took a look at the car and said there is a line coming from the side of the master cylinder, to the proportion valve, and then to the front left and rear brakes, and there is another line straight from the bottom of the master cylinder to the front right (my lefts and rights might be backwards). i have no idea if thats the stock configuration, but it sounds off to me. i have no idea whats wrong with it but im going to the track in a month and i need to get it figured out. any help is appreciated, thank you. and if this is in the wrong section sorry, i wasnt sure seeing as the brakes are from a 99.