Lorn,
Question.  What is the pressure in your low circuit during retraction?  It should be low, perhaps 50 psi.  This is the backpressure circuit doing its job, allowing the extra fluid to bleed back to the reservoir.  Once you are airborne and pressure leaks across from the high side to the low side, it should continue to relieve back to the reservoir.  
I just got back from the airport where I tried to replicate your scenario on my test bench.  I could not get the low side to maintain any pressure while a leak was slowly bleeding across from high to low.  If I let it bleed more quickly, I could hear the fluid squirting back into the reservoir.  My test bench is an exact duplicate of the system in the airplane but with extra valves and pressure gauges to isolate and monitor the particular cylinder or cylinders being tested.   I would be curious to know what your pressure indication is on the low side during retraction.  If it is at or near zero with the gear coming up, you've got a real head scratcher on your hands.
Chris