since it cam from Colorado it the bus may have had a sander option and the valve would have controlled the sanders maybe on each side in front of the drive wheels. Or it may have also controlled tire chains at the rear wheels.
You may see signs of something that may have been removed by the last owner.
In some states there is a "STOP" sign that fols out at the back as well as the front of the bus. I have seen these both air operated but have not seen this particular lever. One bus with the dual "STOP" signs is at http://www.buscrazy.net/upload/5280.htm
Well the new members are getting closer to me anyway..... as for wheelchair lifts most I have seen use hydraulics , not air. And yeah I've seen more than a few since both my wife and my mom will be useing wheelchairs in the not to far future.