Air Brake Manual

Tractor Protection System (closed)
(trailer not charged)
Air from foot
valve
Tractor protection valve (24)
Air from hand
valve
Two-way
check valve
(26)
Control
(service) line
(22)
Air from
reservoir
Trailer supply valve
(mounted in cab)
(28)
To test the proper function of the tractor protection
valve, hook the tractor’s air lines to a trailer, ensure
the vehicle is secure and the wheels are blocked and
release the parking brake. Verify that the system is at
full pressure, charge the trailer by opening the trailer
supply valve, make a brake application and hold it,
(use a hand brake valve if alone.) Disconnect the
control (service) line (there will be a loss of air.) Now
disconnect the supply (emergency) line. Control
(service) air will stop immediately and supply (emergency)
air should continue to be lost, depending on
the system.
Reconnect the supply (emergency) line and again
service air will be lost.
This tests the proper opening and closing operation of
the spring portion of the valve. A tractor protection
valve with a broken return spring will not close the
valve and will cause loss of air during normal brake
applications if operating the tractor without a trailer.
Trailer Supply Valve
This valve (usually a red octagonal button) is mounted
in the cab of the tractor. The driver opens the valve
by pushing or pulling the button, depending on the
type used.
Opening the valve permits reservoir air pressure
to flow through. This air pressure is piped to the
tractor protection valve and then to the supply
(emergency) line.
The valve is spring-loaded and will be held in the
open position when sufficient pressure is reached.
If the air pressure drops to a range between 45 and
20 psi, the valve will close automatically by spring
pressure, opening the exhaust port. The driver can
close the valve manually. This will open the exhaust
port without the pressure being depleted from the
tractor reservoirs. The spring parking brakes will also
be applied.
Supply
(emergency)
line
(21)
43