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 gladhands.

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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.

Figure 42. Typical tractor and trailer charged with air
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6.2.6 ' Trailer Air Tanks

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor. They provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves.

The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal (and the trailer hand brake).

It is important that you do not let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it and you should drain each tank every day. If your tanks have automatic drains, they will keep most moisture out. But you should still open the drains to make sure.

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6.2.7 ' Shut-off Valves

Shut-off valves (also called cut-out cocks) are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

6.2.8 ' Trailer Service, Parking and Emergency Brakes

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before are not required to have spring brakes. Those that do not have spring brakes have emergency brakes, which work from the air stored in the trailer air tank.

The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected.

A major leak in the emergency line will cause the tractor parking valve to close and the trailer emergency brakes to come on. But the brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. Therefore, it is important for safety that you use wheel chocks when you park trailers without spring brakes.

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

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