All of the following apply if the low air pressure warning signal does not work EXCEPT:

Test low pressure warning signal:
Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The low airpressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.
If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.