there is no "pad spring", the piston just relies on the rotor to push the pad back a few thousandenths. i had this same problem on one of my rear brakes. the problem is most likely the seal in the piston is going bad. i ordered a rebuild kit and rebuilt the caliper which took care of the problem. However most people don't recomend trying to rebuild a caliper on your own as it is a real pain in the ass. I actually didn't think it was that bad. Only real porblem was that there was no way to get the dust boot seated properly. I ended up having to seal the boot to the caliper with some JB weld. If you end up buying a new caliper you are better off getting a stock rebuilt/remanufactured one rather than an off brand replacement from a local parts store. Will run you like 100 with the core.