Brake Mystery

1997 Toyota Corolla brake mystery: brakes locks lock easily, must brake in advance "no pedal". Master cylinder replaced 3 times, booster replaced, proportion valve replaced, lines bled 4 times which helps but proble4m returns. Have taken it to 4 different places including dealership, all are at a loss as to what is causing the problem. Has anyone else had this problem? If so, how is it fixed other than trading it in for a new car?
Thanks!