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. In a truck’s air brake system, relay valves get a signal when a driver presses the treadle, which then opens the valve and allows air to enter the brake chamber via air inlet. The diaphragm gets pushed, then the rod, then the slack adjuster which twists to turn the brake camshaft. Next, it moves the disc, wedge or s-cam, which pushes the brake shoes and lining, creating friction. This friction slows and eventually stops the brake drum’s turning, which stops the wheel. 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