Braking System - Park/Emergency Control Valve
Section 250-0190
SM 1940 4-00
2
Port 'C' - Supply to relay emergency valve (rear)
Port 'D' - Exhaust
The valve comprises an aluminium cover and an
aluminium body. The cover contains the actuating
mechanism and a control lever incorporating a spring
loaded locking mechanism which allows it to be locked
in the 'Park' position.
The body contains two graduating valve units, one
upright and one inverse. The output from each unit can
be set by means of adjusters in the bottom manifold.
Incorporated in the body are two air supply ports, one
in the bottom manifold, the other in the rear manifold.
These supply ports provide a common feed to both
graduating units. There are two delivery ports from
each graduating unit, one in the bottom manifold, the
other in the rear manifold.
OPERATION
Numbers in parentheses refer to Fig. 1.
When control lever (1) is in the 'Off' position, the
'upright side' of the valve is delivering zero air
pressure to the brake chambers and the 'inverse side'
of the valve is delivering hold off pressure to the spring
brakes in order to maintain them in the brakes
released condition.
As the control lever (1) is moved to make a
'Emergency' brake application, it rotates the cam (3),
depressing the 'upright side' plunger (9), closing the
exhaust valve (7) and opening the inlet valve (6)
allowing air to pass to the brake chambers. As the
pressure in the brake chambers increases, the air
pressure acting on the upper piston (8) causes it to
move down against the pressure exerted by the
graduating spring (5), closing the inlet valve (6). This
procedure is repeated for further control lever (1)
movement until full 'Emergency' brake position is
reached, when the air pressure delivered to the brake
chambers is maximum.
During this operation, the 'inverse side' of the valve,
which was delivering maximum pressure when the
control lever (1) was in the 'Off' position, is exhausting,
until, with the control lever (1) in the 'Emergency' brake
position the delivered pressure is zero.
When control lever (1) is moved from the 'Emergency'
brake position to the 'Park' position, the cam (3) is
lifted by the control lever (1) and the pivot (2) allowing
the 'upright side' plunger (9) and the 'inverse side'
plunger (4) to lift and allowing the air pressure in the
brake chambers to exhaust. The vehicle is now parked
by the mechanical force exerted by the spring brakes.
REMOVAL
Numbers in parentheses refer to Fig. 1.
WARNING
To prevent personal injury and property
damage, be sure wheel blocks are properly
secured and of adequate capacity to do the job
safely.
1. Switch off the engine, move control lever (1) to the
'Park' position and place the battery master switch in
the 'Off' position.
2. Block all road wheels, open drain cocks on air tanks
and drain air from all three air tanks. Close drain cocks
on air tanks when air is exhausted.
3. Tag and disconnect air lines and electrical
connection from valve body, to aid in installation.
4. Support valve body and remove mounting hardware
securing valve body to mounting bracket on underside
of dash panel. Remove valve body from the dash
panel.
INSPECTION
Numbers in parentheses refer to Fig. 1.
To check the control lever shaft for damage or
deformation:
1. Move the control lever (1) into the 'Park' position.
2. Remove the outer circlip (10), washer (11) and
handle (13).
3. Grip return spring (14) firmly and ease off circlip (12).
4. Remove spring (14) and locking sleeve (15).
5. Inspect control lever shaft.
Note: If control lever shaft is bent then the complete
valve must be replaced.
Courtesy of Machine.Market
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