a mico brake lock should only be used as a temporary holding assist, and not an over night parking brake, and the operator should be instructed as to the proper way to set and release the brakes, the mico-valve should only be turned on OR -off- with the operator applying stopping pressure to the pedal, and not just slap the handle to the released position, to prevent the damage you are experiancing. (also note that the cam on the handle that pushes the plunger is not at it's widest spot with the handle to the fartherst it can move, but it should be at a right angle to the valve, but can move further, depending on how it is mounted.
For MY money, I get rid of the Mico-lock, and fix the parking brake this truck came with from the factory.