Ok I am having issues with the rear drums on the 71 locking up during a hard stop. This is not an emergency stop just one where you put constant hard pressure on the brakes.

Background is I rebuilt the entire rear brake system. Shoes, wheel cylinders, brake lines, axle flex line. The system was gravity bled, pump bled and vacuum bled.

After initial diagnosis I reexamined the brakes to realize I installed the shoes backwards. I corrected shoes and readjusted them. That didn't fix it. So I went back and sprayed the brakes down with brake clean and roughed up the shoes and drums with 80 grit. Test drove again problem not fixed.

So the only thing left I can think of is proportioning valve. The car has disc/drum but had the cutlass 2in shoes/standard rear drums. The rear now has 2 1/2" in shoes/ larger rear drums from a vista. Drums were checked to be true and freshly cut by a local shop. Could the issue be the bigger shoes/drums? Do the cutlass and vista use different rated proportioning valve?

Any help would great on this issue.

 

I had to put a non stock proportioning valve on mine. You may have to as well.

 

I think you just answered your own question.
You have put bigger rear brakes on your car, not modified the proportioning valve and now the rear brakes lock up early.
You have two options (three if living with this is OK with you);
Put the original shoes and drums back.
Fit a suitable proportioning valve.


I suggest putting it back to original, most of your braking is taken care of by the fronts, and the original set up is probably just fine anyway if it's properly adjusted and maintained, which seems to be something you are easily able to do.

 

A third option would be to install very wide, sticky rear tires.

 

I believe I have foudn the issue. The car is still equipped with the factory distribution valve from drum/drum brakes. So whoever installed the front disc brakes did it half arsed because the car originally had a drum/drum mc when I got it which was replaced. My only guess is that the rear drums before were backed off far enough that they wouldnt grab when engaged. So I am trying a correct proportioning valve to see if it fixes the issue. Hopefully I dont have to replace brake lines too.