On long wheelbase trucks and tractors and on trailers, the distance from the brake chambers to the foot valve is too far to cause immediate application of the brake when the foot valve is depressed. This is called brake lag. To correct this situation, a relay valve (Fig. 21) is installed near the rear brake chambers. A large diameter pipe is connected between the service reservoir and relay valve. The air line from the foot valve to the relay valve now becomes a control line that signals to the relay valve the amount of air to be drawn from the service reservoir for faster application of the brakes. A quick-release valve is built in for faster release of the brakes.
Figure 21. Relay valve