problem with brakes

Either they are adjusted too tight or the wheel cylinders are binding making the rear brakes stay engaged all the time

But they have self-adjusters so how could they be adjusted too tight? And surely the wheel cylinders would not be likely to be binding on both sides?

It's just a guess, but I think your problem will turn out to be the emergency brake cable stuck, rusted, snagged or some such thing. This will cause the rear brake shoes to "drag" while you are driving but has no affect on the front brakes. You will probably need to replace the rear shoes as they may be crystallized from overheating. As far as the cable, you may be able to take it off, lube it enough, and work it enough that you can reuse it. BUT, if it was mine, I would just replace it and not worry about it again. Good luck

Thanks fellow oldtimer! Will check it out. I have a workshop manual now so should be able to figure out how they work.

Don't overlook your rubber brake hoses. When they deteriorate, often the lining swells. When you pump the brakes, it forces fluid past the swollen part into the wheel cylinder, but when you release, the collapsed lining acts as a one-way valve and prevents the fluid from traveling back towards the master cylinder - thereby keeping pressure on the wheel cylinders. Enough to make them drag. Over time, fluid seeps back past the collapsed lining, making the brakes act normal. Moral of the Story: When rebuilding your brakes, replace wheel cylinders and rubber hoses, along with your shoes.

Thanks Randy, will check!

Are the brakes self adjusting? Try backing the car up at about 5-10mph and stepping on the brakes rather hard several times, the self adjusters should engage the brakes at the proper tension... if that does not fix the problem suspect the rear axle housing... I once had a 68 Plymouth Fury that had the same problem after changing the rear shoes, after a few miles they got really hot and started smoking, after frustrating search, we removed the rear axles and the housings were ruined, necessitating replacing the entire rear end... the problem was that the old shoes were worn enough to have clearance but the new shoes exposed where an axle bearing was allowed to spin in the rear end and ruin the machining. When the car's weight was set down on the rear end with new brake shoes, the clearance disappeared and the shoes drug.... replacing the rear end immediately solved the problem with the new shoes (we transferred them to the new assembly).... hope the problem is not that extensive