What is the purpose of the modulating control valve (when equipped)?

Question

Accepted Answer

C: To allow gradual application of the spring brakes when the service brakes fail.. When equipped, the modulating control valve allows the driver to gradually apply the spring brakes in the event of a service brake failure. Or if the service brakes have faded to the point that they will benefit from supplemental application of the spring brake.

What is the purpose of the modulating control valve (when equipped)?

Arkansas CDL Air Brakes Test Info