I replaced my rear discs/pads the other day. In the past my o/s rear caliper gave me grief , the lever the operates the handbrake would not fully release. Now I have the same prob again. The piston seems fine , it wound back in normally , but the handbrake lever is stiff and wont really come back when i release the handbrake. I coated in wd40 and moved it about with mole grips etc. Is this part serviceable or do I need a new caliper ?