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

[ERASER5]

The proportioning valve is either out of center or not correct for your K5. The fast easy fix is to get an adjustable proportioning valve and set it for what you need. Trial and error is the way to set it. The valve should have instructions as well. Master Power Brakes has them. http://www.mpbrakes.com/accessories/...-valve-vl3350w

MPBrakes has an excellent Q&A at the bottom of the page.

[grillzy]

Take it to a shop that has older guys that have worked on older wheels. They will fix you up in a jiffy. Newbies and old dont mix well.

[68panelman]

I also have a 1970 k5 blazer, all drum. All drum trucks don't have a proportioning valve, they have a distribution block. that could be where your problem lies, do you have any pics?