Byron,

 

While focusing on your last sentence, remember this: In flight and at any request to retract the gear, there are no external forces on the hydraulic system - i.e. every thing is down and LOCKED.  In flight and issuing a request to drop the gear, gravity, door springs and the gas nose spring are all imputing forces to the hydraulic system - usually enough to unlock any subpart, especially if the dump valve is opened.  This is good when approaching a landing field for any reason requiring contact with earth.

 

Grayhawk

 

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.