The foot valve is usually located closer to the front wheels than to the rear wheels. The longer the distance from the foot valve to the rear chambers, the more time it will take (known as brake lag) before the rear brakes apply. To correct this condition on a long wheel base vehicle, a RELAY VALVE (14) is installed near the rear brake chambers. A larger diameter pipe is connected between the main reservoir and the relay valve. The air line from the foot valve to the relay valve now becomes a ‘control line’. (The air in the control line ‘dead ends’ at the relay valve.) When the foot valve is depressed, the air pressure in the control line acts on the top section of the relay valve, causing the relay valve to ‘relay’ reservoir air directly to the rear brake chambers through the larger diameter pipe. The pressure of the reservoir air delivered in this way will be the same as the control pressure delivered by the foot valve. Releasing the foot valve exhausts the control air to the relay valve, allowing it to cutoff the flow of reservoir air to the rear chambers, in turn exhausting the air in the brake chambers by the quick release feature of the relay valve.