The Brake System:
Trailer Coupling Hoses & Connectors
Two air outlets are provided at the rear of the truck for connecting its brake system to the trailer brakes. One outlet contains a tag with the word "EMERGENCY" printed on it; the second outlet has a tag with the label "SERVICE." Two air line connections on the trailer also have emergency and service tags. When connecting the two brake systems together, the emergency line on the truck must be connected to the emergency line on the trailer. Likewise, the service lines must be connected together. The service line connects the air control line of the truck to the control line of the trailer to control the normal application of the brakes. The emergency line connects the air supply of the truck to the emergency relay valve of the trailer. If the emergency line should break or be disconnected, the trailer brakes automatically apply.
An air shutoff cock is located at each trailer connection outlet on the truck. The cocks must be turned off to prevent the loss of air when the truck brake system is not connected to a trailer brake system.
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 coupling.
Flexible high-pressure 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.
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 air-over-hydraulic cylinder. It consists of a relay section and an emergency section which work together to control the action of the air-over-hydraulic 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 truck's compressed air system to keep its air reservoir pumped up.
The emergency relay valve also directs air pressure to the air-over-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 air-over-hydraulic cylinder. The exhaust opening is also threaded. A drain plug is usually provided in the bottom of the assembly for draining condensation. |