Brake Problem

Greeting to all Benz owners, I am a second owner benzer myself. because of this site I have been able to bring a 83 300DT back to life. (I carted it home from Atlanta). After a few fixes she began to breathe again. So far it has benn running great. I noticed some pulling to the right while braking, so I checked all the way around,sure enough the brakes were worn sensors melted.I replaced the pads on the driver fornt. When I went to replace the passenger front only one side of the caliper would move. I did replace the pad on the side that did move. Next I purchased a caliper and put the other pad on. This is where the problem starts
I have noticed a srcubing sound sound from that tire I checked the install again but found no problem. It sounds like the caliper piston is slow to retract by the time I can inspect it the dang thing has righted itself. The brakes are no longer pulling to either side.

1. Am I dealing with a defectice caliper
2.Worn rotor
3.Collapsed brake line
Which seems to seems to be the likely culprit?
Am I missing something???

Thanks in advanced

Roboman
245K and still ticking