Just a heads up regarding draining air tanks....the very first tank from the compressor (wet tank) is the one that needs draining the most. If bus is equipped with an air dryer, this almost becomes unneccessary...almost! Still, a good idea to drain all tanks periodically and the drain valves shown are very common, cheap, easy to install and can be had from almost any truck parts dealer.
Bob ... I have them on all my tanks ... they work great and no leaking (3 yrs) so far.