Brake Lock-up

HELP!! I am helping out a friend that is having brake problems with her '94 Baretta.. What happens is in reverse the brakes work fine but in drive all you do is tap the brakes and they lock up.. I took off the left rear drum and the brakes look new with nothing broken. Next i un plugged the ABS on both rear wheels and it still does the same thing.. Is it the ABS or something else.. Please help...
Thanx Ryan Greenland

somthing wrong with the piston or airbubble in the line somewhere maybe. Get some brake cleaner and clean all the shit out and see. If the master cylander ever ran dry she could have air bubbles and those can cause really bad malfunction..