Rear brakes locking up! Help.....

[legotech7]

I just got this truck, 1970 c10 short bed. the PO, had just put in new brakes all around, drums on all fours. I took it out for a 20min ride and I felt the rear rear drag a bit, so I stopped in a parking lot and noticed a bit of smoke coming from under neath, couldn't tell if it was from the rear, so I stop for a min. get back in to drive off and I couldn't move forward at all. The rear brakes locked up completely. So pondering what to do next, I sat there for about 30 min. getting ready to call a tow truck and decided to try to see if it would move, and low and behold, we were free wheeling! I drove straight home, and as I'm getting in the driveway, the same thing happened. I waited for things to cool down and took the rear drums off to see If I could see what the problem could have been. Everything on both sides are brand new no real visible wear on the shoes, but I did have to back off the adjustment on the brakes to get the drums off. They were snug but not tight! Now the PO did add a brake booster to the truck and it brakes well, maybe a little too well. Something is making them tight after depressing the brake pedal a few times but not releasing them back, they just start to lock up. Could it be the valve pictured here, might be the problem? Might I need to bleed the system and readjust the brake shoes, and back off the adjusting screw a bit? I'm a bit perplexed at this. I need the brakes to be functional and safe. Sorry for the lengthy post, but I thought a bit of detail was warranted. Thanks to all for any insight...