I have a 68 Chevelle 396 4spd with four drum brakes. The rear brakes will lock up tight after three times pumping the pedal. While restoring the car I put a working “power brake booster” from my 69 chevelle on the car (with new front brake lines and one rear line to the proportionate valve due to the new location of the master). The only way to release the rear brakes is to open the rear brake line bleeder and let in air. I replaced the rear wheel cylinders twice, I used both diameter bores and still same problem for either size. I have bleed the system 700 times, no air. The front brakes work perfectly, stop and release all the time.
The only thing different is the brake pedal is 1-2 inches lower than the clutch pedal. While trouble shooting (shot gunning parts) I found when I disconnect the vacuum hose from the power booster the rear brakes will release, but the pedal is like a rock and is not safe or practical. I was told the master cylinder is the same for power and non-power brakes. The power booster for 68 is same for 69.
Its been a year plus I seem to remember using the stock rod from the brake pedal to the power brake booster. There must be a 68 only rod length or some 68 only booster difference?
I am able to drive the car with the rear wheel cylinder bleeder open but not too safe and I don’t like driving with a helmet on. The front brakes work great. Before I shotgun anymore parts I thought id ask the experts (YOU). I need this fixed so when your down in south Florida I won’t run into you at a red light, Literally!

Sounds like the push rod lengths are to long.
Try adusting it shorter and see what happens.

Or put a shim between the cylinder and the booster and see what happens. It might be a defective booster that is holding the piston in whenever there is vacuum.