power drum/drum brake issues

[az1970k5]

Tried the search but was unsuccessful in finding a similar issue already discussed. I had a shop replace everything imaginable as far as brakes go on my 70 k5 to include 4 new drums, shoes, and master cylinder.. It stops fine, brakes work as they should but when I really get on them even at 15mph the rears lock and slide evenly. When going in reverse and I get on the brakes they do not lock up and slide. I took it back 3 times for readjustments, they even turned a brand new drum.The shop says it's because the rear is alot lighter than the front and that I would need a proportioning valve for the rear brakes in order to cut some of the pressure going to the rear brakes. Is this a common problem with these old trucks and is there another alternative?
Thanks