Page 183 - Driving Commercial Vehicles Manual+

driving commercial vehicles

Trailer with spring parking brakes — dynamited

This diagram shows a broken supply line. The trailer spring brake valve has
sensed the loss of pressure in the supply line and has exhausted the air
pressure from the spring parking brake chambers, causing the spring parking
brakes to apply. Note the burst of air from the exhaust port of the trailer
spring brake valve.

The supply line has broken,
causing the trailer spring
parking brakes to apply.

This action can also be called dynamiting of the trailer brakes. The trailer
brakes will also be dynamited each time the glad hands are disconnected,
or when the driver closes the trailer supply valve that’s located on the tractor
dashboard.

Tractor protection

If the mechanical connection between the tractor and trailer fails, causing
fast fact the trailer to separate from the tractor, the two connecting air lines break. Air
pressure from the tractor system rushes out through the broken supply line,
A peace officer may place and if the driver applies the brake, air pressure would rush out through the
your vehicle out of service if broken control line.
the tractor protection system
allows the tractor pressure to To prevent the tractor air from being depleted to an unsafe level, tractors are
drop below 20 p.s.i. (138 kPa) equipped with a tractor protection system.
without the trailer air supply
valve automatically closing. A tractor protection system consists of a trailer air supply valve located in
the tractor dash, and a tractor protection valve, usually located behind the
tractor cab. All of the supply and control air delivered to the trailer passes
through the tractor protection valve.
If the trailer breaks away, the tractor protection system will automatically shut
off air loss from the tractor, preserving enough pressure for the driver to make
a safe stop.
Some tractor protection systems will shut off immediately in a breakaway, but
some will allow tractor pressure to drop to as low as 20 p.s.i (138 kPa) before
shutting off.

Proper operation of the tractor protection system should be checked as part
of the daily pre-trip inspection.

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