Lspv. Components. Air brake. Air brake chamber. S-cam Air Brakes. The Official Air Brake Handbook. The trailer supply valve is usually a push/pull type valve near the driver that supplies air to the trailer’s brake system through the supply line.
This valve normally has a red, eight-sided knob that the driver pushes in to open and supply air to the trailer and pulls out to close or when no trailer is attached. Failing to close the trailer supply valve when there is no trailer attached allows a large amount of air to escape from the towing vehicle’s air brake system. Air brakes.