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

You answered correctly. Both piston wear in the air compressor and a leak in a compressed air circuit connected to the multi-circuit safety valve can cause the compressor to run continuously. Piston wear reduces the compressor's ability to build up pressure, so the system never reaches cut-off pressure and keeps running. Likewise, a leak means pressure can’t build up properly, which also prevents shut-off. These are typical issues in such braking systems. The third answer is incorrect because, in most cases, a valve that doesn’t switch off would not normally make the compressor run continuously, unless there is another problem related to pressure buildup. Remember: persistent compressor running usually signals pressure loss due to leaks or mechanical wear.