One side fast wearing brake pad

98 Villager 120k miles ABS
I replaced front break pads and driver side rotor 15000 miles ago and now Passanger side pads are worn half way but driver side completely (unevenly, piston side worn more than outside). Yesterday when I was replacing passanger side pads, caliper piston was pushed in easy with a clamp, but driver side piston was pushed in only half way and I had to open bleed valve slightly to push piston in rest of the way.
After replacing pads, I was driving around and break was working fine but smelled hot break. I am thinking caliper piston might not be returning but I haven't take wheel out to look at it yet. I am wondering what the problem might be, can you help me, thanks.