The valve tube
The end of the valve stem just sticks out of this valve tube. With this end you had to pierce the polyethylene sheet that covers the the mushroom like top of the valve tube after (renewed) attaching it to the valve seat on the other side of the board.
The plastic that covers the valve seat becomes the ideal lining for the little valve disk cut from a flip-flop toe slipper.
The good functioning of the Hy2U depends on very careful design work to get get this crucial valve function properly!