Air Brake Manual

Automatic Trailer Supply Valve System
The diagram below illustrates air being piped from
the dry reservoir line to the trailer supply valve (28).
The tractor protection valve (24) is fed by two lines:
one from the trailer supply valve (28) and one from
the two-way check valve (26). Leading off from the
tractor protection valve are two lines, each with a
glad hand coupler (20). These two lines are referred
to as the control (service) line (22) and the supply
(emergency) line (21).
28 26
24
22 21
20
In the diagrams the upper line is the control (service)
line (22) and the lower line is the supply (emergency)
line (21).
Illustrated is a tractor equipped with a trailer supply
valve (28) and a tractor protection valve (24). The
trailer is not coupled and the tractor is being
operated alone (“bobtailing”). The driver has not
opened the trailer supply valve (28) and the hand
valve (30) is closed.
30 28
26
24
22 21
20
31
44