TRUCK SERVICE MANUAL
LIMITING AND QUICK RELEASE VALVE
seat, and the exhaust is closed. In this position, air has
free passage through the valve.
If a foot brake valve application is made, air passes
through the two-way valve and enters the side inlet port
The limiting quick release valve and two-way valve
of the limiting and quick release valve. At the same time
are used as a combination in air brake systems of trucks,
air also enters the top inlet port; therefore, pressure acts
buses and tractors. This combination permits full brake
on the top and side areas of the piston. The piston
valve delivery pressure to the front wheel brakes when
moves down and the exhaust valve seats, closing the
on dry roads, or, at the option of the driver, limits the
exhaust port. At the same time, the inlet valve opens
pressure to the front wheel brakes 50 percent of the
and air passes through and out to the brake chambers.
brake valve delivery pressure when on slippery roads.
When the air pressure beneath the piston and in the
brake chambers equals the air pressure being delivered
to the limiting quick release valve by the foot valve, the
piston raises slightly and closes the inlet valve. Then
when the foot valve application is released, the air on
the top and side areas of the piston is exhausted through
the foot valve. Now the air pressure beneath the piston
raises it and the exhaust valve opens and the air in the
brake chambers is exhausted out the limiting and quick
release valve exhaust port.
Slippery Road Position
When the handle of the two-way valve is in the
Fig. 3 Limiting and Quick Release Valve Combination
slippery road position, the valve is "off". Its inlet valve is
seated and the exhaust is open. Air is stopped from
The two-way control valve (See Chapter IX) is
passing through the valve.
mounted on the instrument -panel within reach of the
If a foot brake valve application is made, air enters
the limiting and quick release valve only at the top inlet
Both the limiting quick release valve and the two-way
port and not its side port, because air is stopped at the
valve are connected to a delivery port of the foot brake
two-way control valve. Air pressure is only on the upper
valve. One air line is connected to the inlet port on top of
inner area of the piston. The piston moves down, the
the limiting and quick release valve. Another enters the
exhaust valve seats, and the inlet valve opens. Air
inlet port of the two-way control valve. A third air line is
pressure enters the brake chambers and also acts on
connected between one of the delivery ports of the two-
the lower area of the piston. This lower area of the
way valve to the uppermost side port of the limiting quick
piston is about twice the size of the upper inner area of
release valve. There are two brake chamber delivery
the piston, so when the pressure acting on the lower
ports and an exhaust port located in the bottom cover of
area is about half of the brake valve delivered pressure,
the limiting and quick release valve.
the piston lifts and closes the inlet. In this position the air
pressure in the brake chambers is approximately one-
The limiting quick release valve, besides providing
half the brake valve application pressure.
for a 50 percent reduction of front wheel braking
pressures, also serves as a quick release valve upon
release of brakes.
Once each year the limiting and quick release valve
and two-way valve should be disassembled and cleaned.
The inlet and exhaust valve boots and piston grommets
Dry Road Position
should be checked and replaced.
When the handle of the two-way valve is in the dry
road position, the valve is "on" its inlet valve is off its