How the valve works
From the inside of the "rubber" polyethylene bag, you can see how it is secured to the board and to the valve seat. Red string to slots cut into both ends of the bottom board keep the bag in place and hold the rubber string that presses down on the valve in the middle.
The big trick is that you can open the valve by pushing up on the other end of a wooden pin (the valve stem) that sticks out from under this board.
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