What can cause the air compressor in a braking system with a multi-circuit safety valve to deliver compressed air continuously?

Your answer is correct. In German braking systems with a multi-circuit safety valve, continuous operation of the air compressor can be caused by piston wear or a leak in one of the connected compressed air circuits.

Piston wear reduces the compressor's efficiency, preventing it from reaching the required cut-off pressure, so it runs continuously. Similarly, if there is a leak in any air circuit connected to the safety valve, the system cannot maintain pressure, causing the compressor to keep running to compensate for the loss.

The multi-circuit safety valve itself switching off is not a cause because it is designed to regulate and stop the compressor once sufficient pressure is reached. Understanding this helps in diagnosing air brake system issues efficiently.