[B and C]

Went to the coach today and the 120v compressor was not working.  After checking fuses I found when I switched on the 12v master switch on the control panel in the coach where the lights, step, etc. (just inside the coach door) are controlled, the pump started right up and pressured the system up.  Is this normal?  If not, where do I start looking for the problem?  I don't recall having to do this is the past.
Thanks