rear brakes

[MYNSBIGR]

I picked up my truck the other day after sitting for 2 weeks and a i drove 20 km after that the rear brakes both left and right started to seize, as i drove on ward to the shop it got worse and worse , at the shop we bled the brakes several times in the rear turning the rear wheel by hand until free then it was fine . i been driving everyday since 200 km a day and today it started again i had to pull over and bleed the rear brakes on the side of the road they freed up pretty well and after supper when i drove home it was fine again. anyone got ideas?
2009 single wheel f250
thanks for any elp in advance i have read some toics but nothing as specific

 

[nswsparky]

sticky caliper, maybe, had that problem in our 250 gasser work truck.