Floor jack will only lift when the foot is weighted?

I was given a very heavy duty, old floor jack. It will lift a vehicle and hold it up all day without losing any pressure(yes, I always use jack stands).

The thing I've noticed is that it will not raise the foot unless the foot is actually weighted. For example, when its fully lowered, I have to step on it while I pump it to get it to raise into position. Once it's weighted, it works perfectly.

Does anybody know the cause or fix for this? I checked the oil, and it seems full.