SERVICE MANUAL
pushing end cap out. Install snap ring (1) into valve body
groove properly.
MIDLAND-ROSS MANIFOLDED TRACTOR
PROTECTION VALVE
DESCRIPTION
The manifolded tractor protection valve (Fig. 3) can
be referred to as a modular tractor protection valve,
since it is one valve that incorporates several different
functions.
The valve incorporates a manifold built into it to
accommodate two (2) brake valve or foot valve ports and
a hand control port through two built in two way check
valves for service brake application. It also incorporates
a quick release feature, automatic shut off on loss of
emergency trailer air and an automatic bleed back of any
trapped service air.
Fig. 3 Midland Ross Manifold Tractor Protection Valve
Location of Ports of Valve Shown in Fig. 3
1
Trailer Emergency
2
Trailer Service
3
Stop Light Switch
4
Brake Valve Secondary
5
Hand Control Valve
6
Brake Valve Primary
7
Tractor Emergency
8
Exhaust
The manifolded valve establishes whether the
service air brake line connecting tractor and trailer brake
systems is opened or closed. The emergency passage
through the valve is always open. The opening and
closing of the service air brake lines is established by
shuttling a spring loaded tapered piston working against
piston forces developed by air pressure levels in the
emergency passage. Manual activation of the valve is
performed by the operator through the use of the Trailer
Supply (red) knob at the instrument panel. When the
Trailer Supply Control is opened (knob pushed in), air
pressure is supplied to the emergency port to open the
tractor protection valve. Pulling the Trailer Supply Control
knob "out" exhausts the emergency line at the air control
valve allowing the tractor protection valve to close.
A quick release feature incorporated in the service
passage serves to improve brake release time response.
During service brake release, the service line air
pressure
in
the
trailer
system
is
exhausted
to
atmosphere at the tractor protection valve as compared
to exhausting at the foot valve or hand control valve.
The automatic shut-off bleed back feature allows the
valve to automatically shut off under a full service brake
application. When the Trailer Supply Control is pulled out
while the service brakes are applied, the bleed back
feature allows the trapped air in the trailer service line to
bleed to "0" kPa or psi pressure.
During normal tractor operation without trailer, the
Trailer Supply Control knob is "out" and the "service"
passage at the tractor protection valve is closed. The
Trailer Supply Control valve blocks the air pressure to
the emergency passage to the tractor protection valve.
Air passages leading from tractor system to trailer
couplings are blocked.
When coupling the tractor to the trailer, the Trailer
Supply Control (red) knob is pushed "in". Air flows
through the tractor protection valve emergency line to
charge trailer brake system reservoirs. Trailer brakes are
released simultaneously if trailer brakes are holding prior
to coupling. When the air pressure in the emergency line
reaches approximately 276-345 kPa (40 to 50 psi), the
tractor protection valve service passage will open.
During normal tractor-trailer operation, the tractor
protection valve remains open, allowing air to flow in
either direction between tractor and trailer air brake
system. Trailer service brakes are applied and released
by operation of either the foot brake valve or hand control
valve, when air flows through the service passage of the
tractor protection valve. Trailer air reservoirs and brake
system are replenished by air flow
through
the
emergency passage.
OPERATION
Refer to Fig. 4 for numbers and letters in parenthesis
throughout this text which applies to the Midland-Ross
manifolded tractor protection valve.
CTS-4079 - CHAPTER XI - Page 6
