Lesson 2/Learning Event 1
A quick disconnect air hose coupling assembly is installed on the air
line connections of both the trailer and the truck. The coupling
assembly contains a lockpin and a replaceable body washer. When the
trailer connections are not being used, a dummy coupling is installed
on the air hose coupling assemblies to keep dirt and water out of the
Flexible highpressure hoses are used to connect the air coupling
assemblies on the truck and trailer together. Hose coupling
assemblies that interlock with the couplings on the vehicles are
installed on the ends of the hoses.
EMERGENCY RELAY VALVE
Trailers that are equipped with their own air reservoir have an
emergency relay valve. This valve is mounted near the trailer's air
reservoir and airoverhydraulic cylinder. It consists of a relay
section and an emergency section which work together to control the
action of the airoverhydraulic cylinder and wheel brakes.
In normal operation, the emergency relay valve serves as a relay
station. It receives air pressure control signals from the truck
brake system and relays them to the trailer brakes. Instead of using
air pressure directly from the truck to apply the trailer brakes, the
valve uses air from the trailer air reservoir. When the brakes are
released, the applied pressure is released through an exhaust port on
the emergency relay valve. The relaying action of the valve speeds
up the action of the brakes.
In addition to the above, the emergency relay valve controls the flow
of air from the truck reservoir into the trailer reservoir. Since
the trailer does not have an air compressor, it must depend on the
The emergency relay valve also directs air pressure to the airover
hydraulic cylinder to automatically apply the trailer brakes if the
trailer breaks away from the truck or if there is a serious leak in
the emergency line.
The emergency relay valve has two main body sections separated by a
relay valve diaphragm. It contains three internal valves and a
number of air passages. Threaded openings are provided for
connections to the emergency air line, service air line, air
reservoir, and airoverhydraulic cylinder. The exhaust opening is
also threaded. A drain plug is usually provided in the bottom of the
assembly for draining condensation.