Caliper with a problem?????
If the vehicle had been sitting for 3 yrs i think that is your first clue and points to a semi - stuck caliper piston imo. When you say you pulled the piston in about 1'4 with a clamp and then pumped it out again a few times , i think that is when you freed up the piston a little. We did my sons old volvo a while ago which was heating up one front wheel and found the rubber boot that covers the caliper piston was split which allowed rain water in . This allowed a rusty build up of sludge between the piston and caliper cylinder. We had to battle to get the piston out. Once we cleaned it out it was fine. We polished the piston and cylinder with 800 wet and dry. We even used the same piston seal as the edges were still okay. This worked for us. You never know, it may work for you.