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The park and emergency brake circuit will hold a fully loaded vehicle on a 20 percent grade without the use of air pressure. It will also apply the brakes on all axles equipped with spring brake chambers if the air pressure in both the primary and secondary service circuits falls below 311 to 138 kPa (45 to 20 PSI).

Parking and emergency brake functions are accomplished through the brakes of the vehicle, which are operated by spring brakes. Spring brakes apply and hold the vehicle's brakes by using spring force rather than air pressure. Air pressure is required to hold the brakes off. This means that air pressure must be available to release the park brakes of a unit before it can be moved. The spring brakes will also be applied if the air pressure falls below 311 to 138 kPa (45 to 20 PSI) due to a complete failure in the system. Control of the spring brakes comes from the park control valve.

The tractor-trailer air circuit

Supply line

The supply line carries air from the tractor service reservoirs, primary and secondary, to charge the trailer reservoirs.

Service line

The service line carries controlled air pressure from the tractor to apply the trailer's service brakes.

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