1975 Chevy Truck abrupt braking

I own a 1975 Chevy C10 Scottsdale that has been sitting for over a year and we've bled the entire brake system since our repairs are complete to get it up and running. It's always had very (touchy) brakes that stop on a dime. My question is why would the right side front and rear brakes tend to lock up and also the rear seems to act as though it's not releasing as it should. When applying the brakes they go so far then abruptly grab. The rears have new drums and brake cylinders installed by Midas before we had to park it for repairs. She's had all brake work done, system bleed, including the front rotors lathed. Can the proportion valve be the culprit? Is it possible that the front caliper on that side is sluggish? I have had a problem with the rear brakes heating up too much so we had Midas rebuild the cylinders. Thanks

Hi, I was finally able to take it for a test run today for 25 miles or so and the brakes seem to be normal once again. I believe that sitting for so long tends to make everything tight and stick somewhat. Midas agreed. I'll check out all parts and hoses though just to be sure and bleed all the old fluid one more time. Hopefully there'll be no more lock ups. Thanks for the help :)