I am designing a hydraulically activated 4/2 spool vlave that will work at relatively low pressures (10 bar max). The through fuid is water and the pilot fluid is also water (filtered at 20 microns) The body will be injected plastic rather than traditional machined or cast metal. The spool would be stainless steel.
With a liquid medium, the gate valve spool would stick in either of the extreme end positions, because the end of the spool and the end face of the pilot cavity would form an effective seal against each other, so pilot pressure would not reach the end face of the spool at all, or would reach it after some indeterminate time delay.  The spool needs to have a 'bump' on each end so the pilot pressure can act on the full end area of the spool.  Machining roughness accomplishes the same thing in a pneumatic valve.
The horizontal split of the sleeve/ housing is very unusual in both air and liquid valves.
I have also seen the rubber/metal rings.  Chances are you'll end up with a custom part, even if they happen to be available in the geometry you need.
Using a seal as a sliding bearing is likely to produce surprises.

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