Well.....is the air dryer functioning properly? Kinda sounds like a restriction somewhere due to ice. Beyond that, everything greased and working properly? No missing springs on shoes, at least the ones you can see? All the brakes adjusted properly? Pedal and associated pivots, pins etc not binding up? Does the truck have ABS? Build up and hold air properly? Relay valve could have trash in it as well, with brakes set is there any air leaking from exhaust of relay valve?
Did this come on all of the sudden or has it be a progressive problem? A good visual inspection will tell you a lot. Chock the wheels, release parking brake and have somebody step on the brake and release. Watch rod stroke from chamber, out smooth, in smooth. Watch shoes and see if they move and return properly. Listen for air leaks. I assume shoes are in good shape and they're not about to cam over or anything like that. Watch the front action too looking for the same things. There's a restriction or something bound up....that's my swag anyway.
