Brakes locking up while driving

I have a Toyota T100 that is having some major brake problems.  We’ve had it in and out of the shop since October and replaced most of the brake system except the master cylinder.  As you are driving along the brakes just lock down tight.  If you let it sit and cool off, then it will work for awhile.  It is getting rather expensive with no solution to the problem.  I would really like to hear some suggestions on what to do.