The fluid circuit is really a closed loop.  The only options for returning fluid to the reservoir are either through one of the pressure relief valves or the low resistance back pressure circuit.  Excess fluid needs to return to the tank every time the rod volume displaces fluid volume.  When the rod volume is removed from the system, make up fluid is pulled in from the tank through a one way check valve.  During retraction, the spool valve opens a low resistance path from the low side to the reservoir, eliminating a large pressure build up in the low circuit during retraction. - which also eliminate the possibility of pressure build up due to thermal influence.

One side effect of the correct spool valve orientation and a leak free system is that you cannot manually push the gear up even with the dump valve open unless the spool valve is moved into the retract position.  After opening the valve you still have a closed loop from high to low that is now bypassing the pump.  When you try to push the gear up you get hydraulic lock.  You are trying to add the rod volume to a closed system and the only route back to the reservoir is through a relief valve.  By bumping the pump in the up direction, even with the dump valve open, you can push the spool valve into the retract position which opens up the low resistance path to the reservoir for the low side circuit.  If you are able to push the gear up at all without bumping the pump, I would suspect the spool valve is backwards.  Let us know what observe in the low circuit during retraction.  You can also try the direction-reversal test during retraction.  The response should be instantaneous.

good luck