ORIGINAL: Aussie Bart
I was talking to a friend last night who was setting up a UMS system for a different project and he made an off hand comment about how stiff the control valve was to move, just like yours. After a bit more dicussion, we came up with the thought (which I had not warned any of you guys about) that the valve may not be mounted on a sufficiently flat surface or that the screws he used to bolt it down are too long.
The threads in the valve are rather shallow and are directly in line with the spool, and if you put in screws too long they WILL deform the aluminium and bind the spool, same goes if you mount it to a plate that is not truly flat, that is why I use carbon fibre plates (sheet from Hobby King).
From my experiance, without the valve mounted you shouild be able to shift the spool quite easily once the seals ''break free'' (stiction on the o rings), when you mount the valve it should be exactly the same. If you get a chance, back off your screws a bit and see if you can move it by hand.
If not, pop the E clip off one end of the spool and pull the spool out, have a look on the spool for any shiny spots, this indicates that the spool is rubbing, if so you may need to wrap a bit of 2000 grit wet & dry paper around a bit of rod just smaller than the bore and try and lap the high spot out of the valve body, just don't get over zelous.
As I mentioned before, the load on the valve spool hardly changes whether fully pressurized or not as the spool is pressure balanced design.