That stainless steel ball is a valve. It allows water in when the spring in the piston makes the piston follow the eccentric but closes the hole when the piston reverses direction forcing water up the water tube. If the head of the piston has a hole in it from the eccentric or is stuck in place or if the spring is broken, it won’t pump water. There should be another ball close to the base of the water line. Both should move freely.
