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Gerard,
As Randy S points out, opening the dump valve
merely equalizes the pressure on both sides. But, it's a much lower pressure and
therefore allows the up pressure switch to stay open even when the fluid
temperature has increased substantially. I shut down the engine, turn off
the master, and then open the dump valve which I leave open while parked.
Remember that the landing gear has an over center mechanism on each gear leg
that locks the gear in the extended position. The gear stays locked until
hydraulic pressure is used to unlock the over center mechanism(s) at the
beginning of the retract cycle. If you were to open the dump valve with
the master switch on, the pump will run continuously but will build no pressure
till the valve is closed. For those of us who occassionally have the pump
not respond to the down switch while in flight, opening the dump valve
momentarilly will reduce the hydraulic pressure that has built up during our
flight through changing temperatures, thus allowing the pump to come on and
extending the gear.
Byron Eskesen
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