John,
This has gone around several times.
The dump valve you have is really a pressure equalization valve.
A true dump valve will dump the pressure to 0. I will explain.

This has happened to me as well. I installed hydraulic gauges which tells the story. When you pull the airplane out of a cool hangar into the warm sun, the system heats up and pressure on both the high and low sides increase pretty much equally. Example: my high pressure (gear up) side may start at 0 and the low side ( gear down) at 500 when in the cool hangar.Pull it into the sun for 30 minutes and both will increase say 700 psi. No kidding! That puts the high at 700 and the low at 1200. If the increase of the system hits 900 or so ( low now at 1400 high at 900)   you have hit the gear up limit pressure switch which tells the system (if it were in transit) to stop raising the gear. In your case, when you try to retract the gear the upper limit just keeps the pump from starting as it thinks the gear is already up because the upper limit has already been hit. In our systems, both sides are closed meaning neither vents to the resevoir when the pump is not running. This changes when the pump is running. If you increase the pressure enough on both sides (again with heat) while on the ground the the pressure on the high side will eventually open the upper limit pressure switch. and the gear will not retract. Sometimes opening the dump valve will help as it equalizes the pressure on both sides. If this equalization results in the high side dropping below the high side limit (appx. 900 psi), the gear will retract. It the equalization does not result in this it will not, thus you have to climb out and crack a fitting somewhere releasing the heat induced pressure increase in the system.
This happened a few times for me and I think I am more suseptable to it as I used small hot rod stainless braid hydraulic lines instead of the factory rubber stuff. I think the factory stuff although much heaver has some flexibity allowing it to balloon a little limiting the heat induce pressure increase. I solved this by replacing the dump valve that does more than  connect the high and low pressure sides. Mine does that but it has an extra line that not only connects the high and low pressure sides equalizing pressure on each side, it also dumps the pressure to 0 with a low pressure return line to the resevoir. If you had my dump valve and low pressure return line, you would have been able to retract the gear by cracking the dump valve and closing it again as you would truely have dumped pressure not just equalized pressure.
I have heard of some doing a bypass on the pressure switch to momentarily get the gear pump to move, for me I like my set up which is similar to the Legacy. I like the idea of being able to release all pressure from the system, not only to solve the issue describe here but to ensure I can always get the wheels down without power as there is no way to keep the gear up if you vent the pressure to 0.

FWIW...
Randy Snarr
N694RS
235/320
--- On Tue, 4/6/10, John Spry <spry@paradise.net.nz> wrote:

From: John Spry <spry@paradise.net.nz>
Subject: [LML] gear retract problems - 320
To: lml@lancaironline.net
Date: Tuesday, April 6, 2010, 5:59 AM