If I depress the service brakes, parking brake engaged, and put the bus in either D or R, a "buzzer" in the dashboard sounds... on/off, not constant........ If I depress the service brakes, release p...
I would check the level in your jacks reservoir in the compartment behind the street side duals. It should be up to the level of the nut you remove to check the level; the jacks must be retracted "up".
If this is ok and you air pressure gauges are reading properly then you have either a bad level sensor on the jacks or a bad airbrake sensor.
I personally start with the level sensor first. Then move on to the airbrakes.