Brake Problem

[swb70]

I have a 1970 chevy pickup and the rear brakes lockup on me when I hit them.
I have to bleed them off to release them. Was told that the problem may be the porportional(spelling wrong) valve. I have never had to replace one of these but guess the can go bad. All the brakes were rebuilt about 5 yrs ago.
Shoes, wheel cylinders, hoses, master cylinder. Could this be the problem. Already tried to buy one at local auto parts and was told that this was a dealer part. Any help will be appreciated.. Thanks Ben

[cdowns]

well the first thing to do is to take the drums off and look for obvious problems like excess rust on backing plates adjuster stuck wheelcylinders leaking shoes worn out grooves in the drums rust on the braking surface of the drums broken or weak springs axel seals leaking etc

[68gmsee]

I had a similar problem that I fought for about a week on one of my drum brakes. Replaced brakes, springs, redid the drums, new brake cylinder kits, etc...

I finally decided to check the lines. On mine the problem was the rubber line above the axle where the metal line attaches to the rubber line and then to the frame. It had swelled/collapsed internally.

Here's how it worked. When you depress the pedal, there's a high hydraulic pressure forcing the fluid through. But when the pedal is released, spring tension forces the fluid back to the master cylinder. This was not enough to force the fluid back through the swelled rubber lines.

This also happens on the front rubber lines. So if you find one that's bad, you need to replace all of them..

Hopes this helps.

[swb70]

Thanks cdowns and 68gmsee. I did rebuild all break parts about 5 yrs ago
except for the flex line near the rearend. Will try that. Thanks again.