Just finished a dragging brake caliper experience with my 911...
Like John said > a dragging caliper means that the brake is always "on"... the caliper fails to retract back into the cylinder > so the brake pad is in contact with the rotor..
Waht to do ??
1. Buy new calipers [co$$$tly] [$300]
2. Buy rebuilt calipers [less co$$tly] [$100]
3. Rebuild the caliper yourself >> actually doable..[$16]
I rebuilt my calipers > was less difficult than I expected , and saved a ton of money..
check out
http://www.pelicanparts.com/techarticles/911_caliper_rebuild/911_caliper_rebuild.htm
although the article is for Porsche brake caliper > they all use the same basic principles...
The rebuild is basicilly cleaning up the inside > piston and cylinder, installing a new seal ...
good to flush your brake system every two years > keeps htings "fresh" on hte inside
kgl