Remove the wheel and brake drum, check the wheel cylinder for leaks, peel back rubber boots, if leaking replace with new (do both sides) clean all fluid from linings (if you dismantle, make a detailed drawing as to where all parts go, including directions of springs. Re bleed brakes. If wheel cylinder is dry, check for damaged hydraulic pipe into wheel cylinder. I will be off line untill tomorrow as I am in Australia and it is after 11-00 PM. Leave a post if you require further infomation.