Most likley going to be a rear brake chamber. These are not good at the best of times. Block the wheels and release the brake and see of both side arms from the chamber have gone back to the release position. Couls also be a caliper as these are not brilliant either.
Some of these had an electric compressor just for the handbrake, These have complicated wiring and were troublesome. Does the handbrake warning light go out?
kerabo, January 2010