The isolation valve uses an internal check valve in the hydraulic system return line to prevent a pressure surge that can unlock the TRAS. The valve bypasses that check valve and connects the TRAS pressure port (TRAS PRESS) to the hydraulic system return port (SYS RTN).
Manually turn the crank to depress the plunger to move the valve to the bypass position. The crank is then pinned to keep the valve in the bypass position. When the pin is removed, the bypass valve is spring-loaded to return to the closed position which pushes the plunger out. The bypass valve prevents a hydraulic lock in the TRAS when you manually extend the thrust reverser sleeve.
Isolation Shutoff Valve

The isolation shutoff valve isolates the TRAS from hydraulic pressure from the airplane hydraulic system.
EFFECTIVITY
BEJ ALL

The isolation shutoff valve is operated by hydraulic pressure from the isolation valve solenoid valve. When the isolation solenoid valve is energized, hydraulic pressure is ported through the solenoid valve to move the slide in the isolation shutoff valve to the open position against spring force. This connects the hydraulic system pressure port (SYS PRESS) to the TRAS pressure port (TRAS PRESS). When the isolation solenoid valve is de-energized, spring force and hydraulic system pressure moves the slide of the isolation shutoff valve back to the closed position. The TRAS pressure port (TRAS PRESS) is isolated from the hydraulic pressure (SYS PRESS) port.
The isolation shutoff valve can be manually locked-out to deactivate the TRAS system. Push-in the plunger to keep the shutoff valve slide in the closed position and install a pin to keep the plunger pushed-in. When the pin is removed, the plunger is spring-loaded to push the plunger out; the shutoff valve slide will remain in the closed position.
Directional Control Valve Solenoid Valve
The directional control valve solenoid valve supplies hydraulic control pressure to open the directional control valve (DCV). When the directional control valve solenoid valve is energized, hydraulic pressure (SYS PRESS) goes through the solenoid valve and goes out the control port (CRTL). When the directional control valve solenoid valve is de-energized, the solenoid valve closes and the control port (CRTL) is isolated from the hydraulic pressure (SYS PRESS).
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TRAS -ISOLATION VALVE

The directional control valve solenoid valve is installed on the isolation valve body with four screws and washers. The screws are lockwired. If the directional control valve solenoid valve is replaced on the valve, there is packing that is replaced on the electrical plug on the solenoid valve. In addition, a solenoid gasket which seals the base of the solenoid valve to the valve body is replaced.
Isolation Valve Solenoid Valve

The isolation valve solenoid valve supplies hydraulic pressure to move the isolation shutoff valve to the open position.
　
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