2
The TW-12
™
valve delivery port is open to its return port
through a passage in the center of the piston.
Parking Brakes Released
When the TW-12
™
control valve plunger is depressed by the
cam action of the lever (Figure 1) the piston contacts the
inlet valve, closing the return passage. Continued piston
movement pushes the inlet valve off its seat in the body,
allowing hydraulic fluid to flow through the valve and out the
delivery port.
SERVICE CHECKS
GENERAL
Read the "General Safety Guidelines" presented in this
manual before beginning any testing or service procedure.
With the TW-12
™
control valve installed in the vehicle brake
system, proper valve operation can only be observed by the
reaction of the system and components controlled by the
TW-12
™
valve. If malfunction or internal leakage is suspected
or specific operational and leakage testing is desired, the
TW-12
™
valve must be removed and bench tested. Bench
testing of the TW-12
™
valve is accomplished using air
pressure regardless of its application or the braking media
used in the vehicle brake system.
OPERATING AND LEAKAGE TESTS
Function test the TW-12
™
valve while it is installed in the
system by starting the engine and warming the vehicle brake
system for 5 minutes. Move the lever cam from one extreme
position to the other three times and observe that the
components controlled by the TW-12
™
control valve function
promptly to the ON then OFF condition. Inspect the exterior
of the TW-12
™
valve. If the TW-12
™
valve is used to control
hydraulic brakes, no fluid leakage should be noted. Pay
particular attention to the area around and beneath the lever
cam.
1. When bench testing the TW-12
™
control valve, attach a
small reservoir volume to the delivery port with an air
gauge installed.
Note: 6-8 feet of ½” tubing or hose could be
used in lieu of a small reservoir.
2. Apply a 120psi air pressure source to the supply port
and move the lever cam to the applied and released
position three times. Note that delivered air pressure is
the same as supply air pressure in the applied position
and that air is promptly exhausted in the released
position.
3. With the TW-12
™
control valve in the released position
and 120psi at the supply port, apply a soap solution to
the return port and around the supply port cap nut.
Leakage should not exceed a 1” bubble in less than 5
seconds (100 SCCM) at the return port. No leakage is
permitted at the cap nut.
4. Place the valve in the applied position and again apply a
soap solution to the return port. Leakage should not
exceed a 1” bubble in less than 5 seconds (100 SCCM)
at the return port.
FIGURE 2 - TW-12
™
CONTROL VALVE HYDRAULIC PARK SYSTEM
POWER
STEERING
GEAR
HYDRAULIC
RESERVOIR
POWER
STEERING
PUMP
HR-1
™
HYDRAULIC
RELAY
TW-12
™
CONTROL
VALVE
MASTER
CYLINDER
HYDRAULIC
BRAKE
BOOSTER
HYDRAULIC
SPRING BRAKE
ACTUATOR
HYDRAULIC
SPRING BRAKE
ACTUATOR
OUT
RETURN
RETURN
RETURN
RETURN
OUT
SUPPLY
DELIVERY
TO SERVICE
BRAKES
CONTROL
DELIVERY
DELIVERY
SUPPLY
IN
IN
IN
OUT
IN
