Several relay valve throughout the air brake system reduce brake lag, thus allowing the brakes to apply faster.

The relay valves in the air brake system speeds up application and release of the service brakes and spring parking brakes on trucks, tractors, and trailers.

Analogy:

I live in British Columbia – my mother lives in Ontario.

If I wanted to send flowers to my Mom, I wouldn’t travel to Ontario, buy flowers and deliver these to my Mom personally – even though that would be her preferred method.

Rather, I would call the local flower shop in Ontario and have them deliver the flower the short distance from their flower shop to my Mom’s home.

Like the delivery of flowers, the air brake relay valve receives a signal from the brake pedal (food valve) telling to deliver 10psi (69kPa) to the rear service brakes.

The relay valve is located in an intermediate position between the air tanks and the service brake chambers.

The relay valve pulls the air the short distance from the air tanks and delivers 10psi of pressure the short distance to make a service brake application.

The distance that the air has to travel to the service brake—like the delivery from the Ontario flower shop to my Mom's house—is shortened significantly, thus reducing brake lag.

The semi-trailer service brakes—the best example—are approximately 60ft (18m) from the brake controls.

A signal is sent to the relay valve in the trailer, and the valve pulls air directly from the air tanks and delivers it the 3 or 4ft (.9 or 1.2m) to the service or spring brake chambers.

Owing to the much shortened distance that the air has to travel from the air tanks to the service or spring brake chambers, brake lag is significantly reduced, and either the service brakes or the spring brakes will apply and release, within a split second of the controls telling the brakes to apply.