James B,
There is a ball check and spring that allows the piston to pull fluid from the reservoir into the pump. On the pump stroke, if this ball isn't seated in its seat, some of the oil will return to the reservoir and the jack will only get a small amount which makes it raise slowly.
I don't know your specific jack, but the procedure is to pull the piston assembly apart, clean any junk out of the tapered seat, and using a brass drift, give the ball a good smack so as to help it form a 'perfect seat' and seal 100%.
It's hard to explain, and it could be as simple as a tiny piece of crap in the seat, but this is probably where the problem lies.
Make sure there is a little vent hole in fill cap too. Good luck.
Stuart
