Page 185 - Driving Commercial Vehicles Manual+

driving commercial vehicles

Two-way check valve

The two-way check valve allows you to apply the trailer brakes independently.
This valve is identical in construction to the one used in spring parking brake
installations, except that it allows the highest application pressure from the
hand valve or the foot valve to be directed to the trailer brakes.

Bobtail tractors

Driving a tractor without a trailer attached is called bobtailing.

Driving a tractor without
a trailer attached is called
bobtailing.

Because a bobtail tractor has very little weight over the rear drive axles, it’s
very easy to lock up the rear brakes, even with a light brake application.
To help prevent this unwanted lockup and to increase control, some tractors
are equipped with a bobtail proportioning system.

This system consists of two special valves: one controlling the steering axle
brakes, and the other controlling the drive axle brakes.
When the tractor is being driven with a trailer attached, the tractor brakes
operate normally.
When bobtailing, the braking pressure to the drive axle brakes is reduced by
as much as 75 per cent, preventing the rear brakes from locking. The steering
axle brakes receive full application pressure.
A tractor with a bobtail proportioning system will stop in a shorter distance
and control will be increased, especially on wet or slippery road surfaces.

Because the steering axle brakes are doing most of the braking, a higher than
normal pedal pressure is required.

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