This is the source of many of our behavioral differences between planes of different vintages.
<<- when hydraulic pump is off it shut off the high and low pressure lines too>>
The most recent 320/360 pumps do not shut off both high and low sides when shut down. The most common spool valve has an o-ring with a pretty tight fit. With the pump shut down, the only force acting on it is a tiny little return spring for the poppet valve. The spool valve thus stays put in its last commanded position and holds open one circuit - the opposite of the one last selected or pressurized.
There are, however, spool valves without o-rings. These can more easily slide back to center and lock both up and down circuits (see Lorn's post about having pressure in both the high and low sides at the same time).
So, if you need to push the gear up by hand, you need to first unlock the down-side poppet valve to allow the excess fluid to return back to the tank. Even with the dump valve open, a quick burst by the pump in the retract direction will slide the spool valve over to the retract position eliminating the hydraulic lock condition. Some time ago I made a video showing this hydraulic lock and how bumping the pump to slide the spool valve into the up position opened a path for the excess fluid to escape.
http://www.n91cz.com/VideoClips/Push_Gear_Up_Hydr_Lock.WMV
Chris Zavatson
N91CZ
360std
www.N91CZ.com