TOUCHY brakes

[chris015]

87' r10. power brakes. discs on front.

Picked it up and its been sitting.

if you BARELY press them on a roll up to a redlight etc the back driver side wheel locks up. I picked up front pads,and calipers. Pulled the drum off everything looks fine. no weird wear. they need to be adjusted a tad tighter,but thats it. No leaks on cylinders. No rust.

Rear center hose has been replaced at one time,some idiot used vice grips and rounded the head off. Ive got a new hose but,i dunno if i wanna change it now or not.

Im wondering if the front calipers are about seized as truck sat up for some time. lil pad left on front too. maybe a worn proportioning valve? checked the reset thing in it,it wasnt flipped. Any help is nice. Im no noobie. Just brainstorming on this. Master cylinder looks to be somewhat new.