Page 180 - Driving Commercial Vehicles Manual+

chapter 8 — air brakes

There are two basic types of trailer air systems — those that use spring
driving tip parking brakes and those that don’t. Although most current trailers use
spring parking brakes, some older trailers and converter dollies in use aren’t
When not in use, glad hands equipped with spring parking brakes.
should be fastened to dead-
end (dummy) couplers or to All trailer systems must have an emergency stopping system that will fully
each other to prevent dirt apply the trailer brakes if the trailer separates from the tractor.
and debris from entering the
air lines. Trailers that aren’t equipped with spring parking brakes use a device called a
relay emergency valve. If this valve senses that the trailer has broken away
Glad hands should also be from the tractor, it applies the trailer service brakes with full trailer reservoir
secured to prevent the lines pressure. This action is called dynamiting the trailer brakes.
from chafing against other
components or bouncing off Trailers equipped with spring parking brakes use the spring force to apply the
the vehicle. brakes (dynamite the trailer brakes) if the trailer breaks away from the tractor.

Trailer with relay emergency valve — charging
This diagram shows a trailer equipped with a relay emergency valve. Air is
passing from the tractor through the supply line to the relay emergency valve,
filling the trailer reservoir.

Compressed air from the
tractor flows through the
supply line to fill the trailer
reservoir.

Trailer with relay emergency valve — applying

This diagram shows a normal service brake application. A control signal from
the tractor has been sent through the control line to the relay emergency
valve, which reacts to this signal in exactly the same way as the tractor relay
valve previously described.

When the driver makes a
brake application, air flows
through the control line.

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