I have a '71 Mav w/4-wheel drum brakes, and both rear brakes seem to lock up immediately with even the slightest pedal pressure.
The Chilton manual I have seems to suggest that this would be caused by grease on the linings. However I've removed the drums and can't see any sign of a leak anywhere.
It's my understanding (correct me if I'm wrong) that proportioning valves were only used on systems with disc brakes, but is there a different control valve that may be defective, that would cause the brakes to lock up?
Had the same problem on a pickup once, everything looked great, no leaks, grease, nothing. After many frustrating evenings of work decided to replace wheel cylinders just for spite. That was it, cured the lock up. Never got into the old ones to see what was wrong, just heaved them into the scrap pile and that was that. Maybe yours are in the same shape, even if that is still not helping, at least you will have two new cylinders on the rear. Not that exspensive anyway. Humm' could be do for new master cyl too. Heck go ahead and put new cyl on the front besides(new shoe's and drum turning while your at it). As the old saying goes "Your life may depend on it" Good Luck as always.
Upon further inspection, the outside edge of the brake shoes (the side of the shoe, not the lining itself) seem to have scored the inside of the drum. IIRC, the previous owner claimed to have done a brake job on this vehicle. Is it possible they were sold the wrong size shoes?
I have checked out the wheel cylinders and they appear to be in good condition, however I may decide to replace them anyway. Thanks for the suggestion, Old Guy.
Yes there could be wrong shoes installed. Look at the mounting hardware to see if it is on incorrectly. You need to see another car to tell weather this may be a problem if not familuar with how it should look. There was another owner that had a severe pull on his front, and turned out to be hardware related.