My 'Cuda wants to lock up and slide way too easy....
The first thing to do is isolate which wheels lock up first and under what circumstances.
The take off the drums in question and look for evidence.
Chances are good something is grabbing.
A shoe that's not square, linings that aren't arced to the drum (tips grab), wrong shoe, etc etc.
Between the FSM and the 1969 Tech on Drum Brakes, there's pretty good factory info on diagnosing.
if the fronts are locking up - then a good chance the pistons are not moving evenly.
My only thought is proportioning valve???
If the rears lock up after the weight has transfered to the front, then yes that's possible.
If you need a review of what proportioning valves do, again Chrysler Tech is a good place to turn.
1967 Chrysler Imperial Disc Brake System From the Master Technicians Service Conference Session 233
