Two possibilities :
1. one or both handbrake cables is binding, probably through corrosion of the cable - solution change cables.
2. the return springs mounted on the rear calipers which should help to release the handbrake are no longer strong enough to work effectively - so far the only solution I'm aware of is to replace the caliper although that seems a bit drastic
it's a fairly easy job to disconnect the handbrake cables at the caliper so you can decide which of these is the problem.
Malcolm, April 2009
