Starting with the trailer is the best way to understanding the principals of tractor-trailer air brake systems. Once you understand the trailer system, it becomes simpler to understand how many of the components used to pull a trailer are the same found on the tractor, such as:
- Air chambers
- Air reservoirs
- Control valves
- Foundation brakes
An air compressor is the only primary component found on the tractor that is not used on the trailer. The trailer system depends on the tractor for two important functions:
- The compressed air from the tractor is used to fill the trailer reservoirs
- The commands to apply and release the brakes are sent from the tractor to the trailer
There are two air line connections between the tractor and the trailer. To actuate the breaks on the trailer the supply line (or emergency line) provides the trailer reservoirs with air at full tractor reservoir pressure. The control line (or service line) is what sends the signal from the tractor to the trailer, telling the breaks when to actuate and with how much pressure. Because tractors and trailers are frequently disconnected and reconnected when switching tractors or dropping (parking) trailers, the airlines have special quick coupling called “glad hand” couplings which must meet SAE and D.O.T. specifications. They are used to disengage and reattach the airlines and are referred to as glad hands because both portions of the connections resemble hands making a handshake. Glad hand couplings have an aluminum body with a steel face or are made of cast iron. The steel face style is zinc yellow dichromate with an aluminum body that is chromate conversion coated for corrosion resistance. These connections are typically color-coded blue and red, with blue signifying the control line and red the supply line.
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