Rear Drums slowly locking up when driving

Just started driving my 1970 440-4BBL Challenger and discovered I have a rear drum brake issue. The brakes worked fine for about 8-10 miles, then I noticed the engine was laboring when I was pulling into my driveway. I barely made it to the garage door when the rear drums locked up. I could smell brake lining material, and check the rear drum temps with an infrared lazer gun and one drum was 290 degrees and the other was 310 degrees.
I figured is was an adjuster issue and let the car sit overnight. The next morning, I was able to roll the car easily with just the push of my hand. Took it out for another test ride and the problem happened all over again. My guess is that it is a hydraulic issue, and that the brake pressure slowly bleeds off from the wheel cylinders. The car is new to me so I don't have the history but it looks like it has a new Master cyl.

Pushrod too tight in the MC.
Needs to have a little play to grow with heat.

internally collapsed rubber line that feeds the rear. Best bet is to loosen a rear bleeder after the brakes start to drag to see if it's a hydraulic issue.

^^^ What Stump said & there just was a comprehensive thread on a pretty much identical situation (2 long shoes on one side iirc).

^^^^^^^^^^^^^^^^^ x3

A little more involved but, crack ableeder loose and see if the brakes release. If so it's a hydraulic problem.

Get it to do it again, crack the rear line loose at the master. Do brakes release? If so problem it s master or booster related. If not then hose, line or p valve related. Duplicate symptoms and crack line at various places, note the results.

Just went through this with a friend...


JUNK 'aftermarket' proportioning valve...


Locating and installing an OEM valve solved the problem...

This seems to be a good approach. I will try and report back. The proportioning valve is stock and original but the MC is aftermarket...

Water in the brake fluid will cause this also, as the boiling point isn't as high as brake fluid and it expands when hot, brake fluid being hygroscopic absorbs water over time so needs to be changed at times to keep rust out of the system and things working correctly.

Seems no one has said if it has self adjusters to check them.If backwards the brakes will adjust up everytime you use them going forward.Rocky

Except they don't unadjust when sitting overnight

But they are released once they cool off, Adjusters will stay tight ..

sounds like they are adjusted a little to tight and drag a little,the more you drive the more they heat and drag,when they cool they retract