Are you having a problem with the handbrake or the balance between the rears on the foot brake.

If it's the handbrake make sure the linkages are free and the auto adjuster is working. Also make sure the cables are free.

If its a balance problem pressing the foot brake then maybe one of the load proportioning valves has gone faulty. If it has seized or the link arm has seized then this will prevent fluid flowing to one of the rear brakes.