Customer stated the driver was getting a message on the dash stating
Park brake air low, brakes may drag. The low air buzzer would come on, yet still maintain 125 PSI. There were no fault codes. If the driver did a very hard stop, the air buzzer would shut off.
The resolution ended up being to replace the brake drag switch. This switch can be tested. It is Normally Open at 0-79 PSI and closes at 80 PSI and higher. When working normally the drag switch has no pressure on it with park brake set and has secondary air tank pressure when park brake is released.
The brake drag switch is located on the switch manifold, which is behind the throttle pedal under a small panel held on by three screws.