A Tether Ball Pump is made up of two parts. A vertical chamber with a reservoir above it, and a plunger underneath the reservoir. The vertical part houses the ball pump, which looks like a little steel egg-shaped thing with an opening at its bottom. When you press on the lower foot switch, air goes into this chamber through that hole, then rebounds back up through the opening in the top of this Egg until it can go out again. As air moves downwards through this tube it pushes on the ball inside, causing it to move upwards against gravity.
Ball pumps are designed so that there’s no sharp edge or crease where one part meets another anywhere along their length – if you find one with any sort of flaw in this respect, please remove said spring immediately – they’ll self destruct very quickly… And not very helpfully too! Some have breathing holes drilled all over ’em for extra breathing ability possibly? Or perhaps they’re used in some other way we don’t know about?
The ball inside is called an o-ring and has markings on its surface so you can measure how far down it is and therefore how hard air pressure needs to be pumped before the ball sits firmly enough without your fingers getting pinched or squashed? Or maybe not? We really do not know! So either open them carefully with tweezers or use an Air Probe (see below) to see what’s going on!