As air pressure in the secondary circuit drops to approximately 35 psi, what is the action taken by the spring parking brake control valve?

When the air pressure in the secondary circuit drops to approximately 35 psi, the action taken by the spring parking brake control valve is to exhaust the remaining air from the secondary circuit. This is a safety feature designed to ensure that the parking brakes engage in the event of a significant pressure loss.

By exhausting the remaining air, the control valve allows the spring brakes to activate automatically. This process helps prevent the vehicle from rolling away or becoming uncontrollable if there is a loss of air pressure due to a leak or failure within the air braking system. The activation of the spring brakes occurs in a manner that secures the vehicle safely, providing a crucial safety mechanism for drivers and others on the road.

Other options related to increasing air pressure or sending warning signals would not be appropriate in this situation, as the primary function of the valve in low-pressure scenarios is to ensure that the brakes are engaged for safety rather than maintain pressure.