Air Brake Manual

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Simple tractor-trailer system

In Figure 42, the trailer has been coupled to the tractor and the service and supply lines of the units have been coupled by using glad hands.

The trailer has a reservoir installed. This tank provides a volume of air near the trailer chambers for normal or emergency braking. The tank is equipped with a draincock.

A relay emergency valve is mounted on the trailer reservoir. This valve can also be mounted directly on the trailer frame near the brake chambers. The relay emergency valve serves three main functions in the system:

  1. The relay part of the valve relays air from the trailer reservoir to the trailer-brake chambers during a brake application. This part of the valve operates like the relay valve previously discussed. It also provides a quick release of the trailer brakes.
  2. The emergency part of the valve directs trailer reservoir pressure to the trailer brakes causing an emergency application sometimes referred to as “dynamiting.” This action occurs automatically in the event of a ruptured or parted supply line between tractor and trailer, or loss of air from the main reservoir system. You may operate the cab-mounted trailer-supply valve to cause an emergency application of the trailer brakes.
  3. The relay emergency valve has a one-way check valve that stops air in the reservoir from going back to the source of the supply. The driver has opened the trailer-supply valve to allow main-reservoir air pressure to be directed through the tractor protection valve to the trailer. Air pressure passes through the relay emergency valve to the trailer reservoir. Pressure builds up in the trailer reservoir to the same pressure as the main reser voirs on the tractor. This is known as “charging” the trailer system. The trailer-supply valve remains in the open position when pressure has built up to between 20 and 60 psi (138 and 413 kPa), depending on the make.

    Drivers can check the operation of the relay emergency valve by closing the supply valve on the tractor or by disconnecting the supply line between the tractor and trailer with the supply valve in the open position.
Typical tractor and trailer charged with air
Figure 42. Typical tractor and trailer charged with air
Click for larger image.


Rev: 2018