1-11. Air Brake System
1. Ratio relay valve
4. Reserve reservoir (XM1063)
7. Air chamber
2. Front reservoir
5. Front relay valve
3. Rear reservoir
6. Rear relay valve
When the gladhands (8) are connected between the towing vehicle and the semitrailer, air shutoff valves on
the towing vehicle are opened.
Air flows through the emergency air lines into the ratio relay valve (1). The air from the emergency air lines
flows from the ratio relay valve (1) into the front and rear air reservoirs (2 and 3). From each of these reservoirs the
emergency air to each relay valve (5 and 6). The emergency air pressure is built up in the semitrailer to equal the air
pressure on the towing vehicle, 90 - 100 psi (630.5 - 758.5 kPa).
When pressure is applied to the brake pedal on the towing vehicle, air pressure is directed through the service
air line to the ratio relay valve (1) and to the two relay valves (5 and 6).
The two relay valves release compressed emergency air to the service brake sections of the front air