PREVOST H3-45 2023 Operator's Manual page 83

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-1652 Operator's Manual H3-45 Coach
Controls and Instruments 4-27
STOP telltale light