Proportioning Valve Problems

Hi i took my 68 barracuda out for a ride today the brakes felt real strange hard as a rock brought it home and the back brakes were smokin needless to say they were lockin up,any way i bled them out no help i finally just release the press :roll: ure off them by cracking the rear bleeders, got it to roll, can some one tell me were i can find a factory proportioning valve or a after market valve that will work any ideas will sure be a great help,Thanks for any input