The small solenoid valve that is electrically operated to relieve pressure on one side of the diaphragm and cause the operation of a larger diaphragm valve.

A Pilot valve is a dead man's switch which requires a human operator to hold it in the mid-position and take some action if the operator takes off pressure or applies full pressure. Often (especially in heavy rail applications) this type of fail-safe braking is provided with one foot and one hand control to reduce driver fatigue by giving them the option to switch between the two.