brake problems
 

Hi every one i am new to the page i am having a problem with my brake locking up in the front its disc brakes after i drive about 15 miles when i slow down its find but once i take me foot off the pedal to move up a few inch's it seem to grab front bakes hard all most like it want to lock up at first i thought to machine the rotors and put organic pads on it that didn't work then i cleaned and set rear brake up plus set up hand brakes it didn't make a different either i checked brake fluid it was a little bit discoloured the car was restored back in 2011 and was finished in 2014 i have been told my problem is a hydraulic problem and could be my master or flex lines or proportioning valve need help as were to start first so help is need my name is Doug my car is a 1972 442 real deal with to build sheets from factory plus G.M paper work look forward to any information!!!