Pull the valve out and submerge in fluid. Pull the jackupwards manually and overfill with fluid. Let it sit with the valve closed and the bleeder open and facing upwards for a period of time. Later slowly bring the jack to its rest position letting the fluid spill from the bleeder.
Worked for me a couple times. Dilute a little brake fluid in it to lossen the seals it it might help.
legend in my own mind
Advice is only as good as the person applying it--
Don't make me give you bad advice!
Lemons or lemonaide, whatever your taste provides.