Yep, they're spring loaded to the closed position. Grease, or in your case fluid forces the ball back against the spring and fluid can flow into the reservoir. The problem is when the fluid level drops, air must enter to replace the missing fluid. If that doesn't happen, a vacuum is formed in the reservoir. The fluid flows to the brake cylinder via gravity. What you have is the same as dipping a straw in water, putting your finger over the end, then removing the straw from the water and watching in amazement as the water stays in the straw. You've just been lucky. Best to replace the zerk with a vent.
Tony