PREVOST H3-45 VIP Owner's Manual page 75

Front Brake Air Pressure Gage (Secondary System)
Indicates the front brake air system pressure in psi. The normal operating
pressure is from 122 to 140 psi.
The low air pressure indicator LED on the gage and the STOP telltale light
illuminate when the front brake air system (secondary) pressure drops
below 85 psi. An audible alarm will sound.
WARNING
The driver is responsible for monitoring the pressure as part of a regular
sweep of the instruments.
If the pressure drops in the secondary system but remains normal in the
primary system, the front axle service brakes will not function but the
drive and tag axle service brakes will operate normally, although if there
is a leak in any pneumatic system (Sec/Pri/Park/Acc), the primary
system might eventually lose its air pressure as well, depending on the
nature and size of the leak. In the event of any air pressure loss in any
system, the driver should pull to the side of the road as soon as can be
safely done and investigate the situation.
Do not drive the coach when the brake air pressure is low.
PA-1638 Owner's Manual X3-45 VIP
Controls and Instruments 4-27
STOP telltale light