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Relay valve
A relay valve is an air-operated valve typically used in air brake systems to remotely control the brakes at the rear of a heavy truck or semi-trailer in a tractor-trailer combination. Relay valves are necessary in heavy trucks in order to speed-up rear-brake application and release, since air takes longer to travel to the rear of the vehicle than the front of the vehicle, where the front service brakes, foot-valve, parking-control valve, and trailer-supply valve (if applicable) are located.
Without relay valves, it would take too long for sufficient air to travel from the brake pedal valve to the rear of the truck or trailer in order to apply the rear service brakes concurrently with the front service brakes, resulting in a condition known as brake lag. To correct this condition on a long-wheel-base vehicle, a relay valve is installed near the rear service brake chambers. In tractors as well as straight-trucks, a remote air-supply is provided in the form of a large diameter pipe connected between the primary reservoir and the relay valve for remote service brake application.
==Trailers==
In trailers, this remote air-supply is in the form of a tank, which is charged whenever the emergency brakes are released via the red trailer-supply valve on the dashboard. In a dual-circuit air brake system, this tank actually receives its air from ''both'' the primary and secondary reservoirs of the tractor; the air from both of these reservoirs is merged via a two-way check valve. The two-way check valve is a pneumatic device that has two inputs and one output; each input is connected to one these reservoirs. Only the air that is at the higher pressure is allowed to pass through to the check valve's output, which then passes through the tractor-protection valve, and then travels onward towards the trailer's air-tank and spring brake valve via the red trailer-supply line (a.k.a., the ''emergency'' line); this releases the trailer's emergency brakes (a.k.a. spring brakes). The tractor-protection valve is a device that prevents air from being lost from the tractor's braking system in the event of the air-lines becoming separated or broken. The tractor's air-lines connect to the trailer's air-lines via metal connectors known as gladhands. The merged air from both reservoirs of the tractor prevents air-loss from only one tractor braking circuit from causing the trailer's spring brakes to automatically apply. This gives the driver more control, and prevents the vehicle from grinding to a halt in an unsafe location, such as in the middle of an intersection.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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