Lesson 3/Learning Event 1
FIGURE 18. RELAY VALVE.
Another air line connects the brake valve to a relay valve at the
rear of the vehicle. Normally, this line also contains the stop
light switch. A second air line supplies reservoir air pressure to
the relay valve. When the brake pedal is depressed, the brake valve
meters a certain amount of air to the relay valve. This air opens a
valve in the relay valve and allows a small amount of air from the
reservoir to go to each of the brake chambers for the rear wheels.
When the brake pedal is released, the air pressure in the line
between the brake valve and relay valve is released. This closes the
valve in the relay valve shutting off the air supply from the
reservoir. At the same time, a quickrelease valve in the relay
valve opens and allows the air pressure in the brake chambers to
escape. This, in turn, releases the rear brakes. The air in the
line from the brake valve escapes from the brake valve exhaust valve.