The large plunger will have a seal at the bottom. It sounds like it is leaking. Normally as the large stage goes up the oil above the seal in the area between the bottom seal and the head will be forced inside the large plunger and push up the small stage at the same time. This works because the area of oil between the outside of the large plunger and casing is equal to the area below the small stage. Over time this will get out of synch. That is why there is a "check" valve in the bottom of the large stage. When lowered all the way down, it allows the small plunger to equalize back to the bottom. You will need to pull the large stage out and inspect / change the bottom seal. Good luck!
