Air Brake Manual

Charging the Trailer System
28
24
39
In the illustration, the compressor has raised the
reservoir air pressure to maximum.
The driver has opened the trailer supply valve (28) to
allow the reservoir air pressure to be directed through
the tractor protection valve (24) to the trailer. The air
pressure passes through the relay emergency valve
(39) to the reservoir on the trailer. Pressure will build
up in the trailer reservoir to the same pressure as
the reservoirs on the tractor. This is known as
“charging” the trailer system. The trailer supply
valve should be open when the tractor pressure has
built up to approximately 90 psi, depending on the
valve.
Foot or Hand Valve Brake Application
26
24
30
31
This and the next illustration are to only show which
brake components and lines are used for a foot valve
and hand valve application.
· Application with the foot valve (31) in orange.
· Application with the hand valve (30) in brown.
Orange and brown are used to show where air is
applied and does not represent the source of the air.
When the foot valve is depressed, the application air
will be applied to the tractor brakes and the trailer
brakes together. As previously explained, the
two-way check valve (26) has shifted, and application
air is being directed through the tractor protection
valve (24) to the control (service) line. If the foot valve
is released and the hand valve is applied, the shuttle
in the two-way check valve would shift and
application air would apply the trailer brakes only.
47