If you have a Williams air throttle the switch is most probably the high idle switch. Those were famous for not working correctly after a few years.
Since prolonged idling is a no--no in almost every urban area when the fast idle switch stopped working they were rarely ever fixed.
The other switch, if it turns on a light might be for the transmission retarder.
Every one of your air tanks should have an air drain. It might be an automatic type or a manual type of drain. But there should be a drain on every tank.
Most newer buses have automatic spitter valves that help to keep tanks dry and/or an air dryer with a dessicant to dry out the air before it enters the tanks. Regardless of what type of dryer you have, you should manually drain every tank on a regular basis. You really do no want any moisture to get into your brake valves, brake air cans, or into any of your air operated accessories.