just want to agree with the above post that you probably have either a split tank or two separate tanks.
most likely on your air guage, assuming you have a pretty modern bus, that there are two needles that usually move in unison, the needles are commonly orange and green i'm pretty sure. One needle is for the primary and the other is for the secondary system. if you loose air in one, you can use the second one as a backup to help get the bus stopped. if both run out of air, then the springs in the rear brakes should lock up the rear tires.