Page 181 - Driving Commercial Vehicles Manual+

driving commercial vehicles

The relay emergency valve has drawn air from the trailer reservoir and
delivered it to the trailer service chambers at approximately the same pressure
as the control signal.

On highway trailers, one reservoir and one relay emergency valve are used
for single or tandem axles. Some tandem logging trailers are equipped with a
reservoir and a relay emergency valve for each axle.

Trailer with relay emergency valve — dynamited
This diagram shows a broken supply line. The relay emergency valve has sensed
warning! the loss of pressure in the supply line, and has delivered full trailer reservoir
pressure to the service brake chambers, dynamiting the brakes. The trailer
Small leaks in trailer systems brakes will remain applied as long as pressure is retained in the trailer reservoir.
without spring parking
brakes can reduce or deplete
the applied pressure,
possibly allowing a parked
trailer to roll away.
Always block the wheels of a
parked trailer so it can’t roll.

The broken supply line has
caused the trailer brakes
to dynamite.

The trailer brakes will also be dynamited each time the glad hands are
disconnected, or when the driver closes the trailer supply valve that is located
on the tractor dashboard.
Motor vehicle safety standards require these systems to remain applied for a
minimum of 15 minutes.
It’s important to follow proper procedures when coupling a tractor to a parked
trailer to prevent the trailer from moving and possibly causing damage. Coupling
procedures are detailed in chapter 5, skills for driving trucks and trailers.

Trailer with spring parking brakes — charging
This diagram shows a typical trailer system that uses spring parking brakes for
parking and for emergency (breakaway) stopping.

This trailer is equipped with
spring parking brakes.

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