I have a 96 opel astra estate in very good condition, except for one thing that keeps getting the better of me. Every so often especially after a night of rain, (I've noticed) when I release the handbrake the back brakes get jammed. I've taken the wheels off several times and cleaned the inside of the brake drum but all to no avail. The only thing that seems to work on a temporary basis is a hefty wallop of a lump hammer. Can anyone tell me what is causing this..? Thanks...