Lesson 3/Learning Event 1
Some wheeled vehicles require special controls on the braking system.
If we consider a 4x4 truck tractor, for example, we know that its
primary job is to pull a semitrailer. Under normal driving
conditions when the road is dry, the braking system will work well.
When the road is wet and slippery, though, the tractor and trailer
will jackknife if the front wheels of the tractor brake too much. To
prevent this from happening, the front quickrelease valve contains a
limiting valve. A twoway valve is also provided to control the
The twoway valve is mounted in the vehicle cab where the operator
can reach it easily. The valve has dry and slippery road positions.
An air line connects the valve to the brake valve and another line
goes from the twoway valve to the combined limiting and quick
release valve at the front axle.
When the valve is in the dry road position, no air can pass through
the twoway valve to the front axle. However, when the valve is in
the slippery road position, air from the brake valve passes through
the twoway valve and onto the quickrelease valve. When this air
acts on the limiting device in the quickrelease valve, it only
allows a small amount of air coming from the brake valve to go to the
brake chambers. This will reduce the amount of braking action on the
front wheels and prevent the front brakes from locking which would
cause the tractor and trailer to jackknife.
Trucks and truck tractors that pull heavy trailer loads must also
have a control that will allow the operator to apply the trailer
brakes separately from the truck or tractor brakes. This device is
known as an independent trailer control valve and is usually mounted
on the truck steering column. Notice in the paragraph above that the
valve is connected to reservoir air pressure through the manifold
fitting for auxiliary devices.
The hand lever on the valve allows the operator to apply line air
pressure to the doublecheck valve at the rear of the vehicle. This
valve allows pressure to be applied on the trailer brakes through the
trailer service line connection.
Notice that the doublecheck valve also has a line coming to it from
the brake valve. In this way, the trailer brakes can be operated
when the truck brakes are applied or they can be applied alone with
the use of the trailer control in the cab.