Just in case any of you have forgotten!!!!!
"Third oil line valve".
With this type of propeller, the pitch lock is unlatched by the admittance of pressure oil into the lock operating (Third ) oil line.
The admittance of pressure oil into this line is governed by a (Third) oil line valve in the controller unit, which isolates the pressure side on the unit pump from the lock operating oil line. The (third) oil line valve consists of a double- headed valve on a common stem, together with their respective seatings.
The double-headed valve stem abuts a small piston. With the valve in its normal position, the lock operating oil line is isolated from the pump pressure side of the controller unit by one valve which is closed on its seating, but opens to the low pressure drain side of the controller unit via the other valve which is of its seating.
A lock-withdrawal solenoid incorporated in the controller unit allows the (third) oil line valve to be hydraulically moved to admit pressure oil into the lock operating line.
There, why did we have so much trouble understanding that in the first place???
Regards.........