Here's how to operate it:
To run it, you need about 5' of feed water pipe for every 1' of fall below your pond or creek water surface. I have 10' of fall and 50' of feed pipe supplying 3 gallons/min. (A little longer would be better say 5.5:1)
Now manually push (cycle) the valve stem up and down about 30 or 40 times to fill the pressure tank. These pumps need back pressure in the tank to operate automatically. When the pump starts to cycle on it's own, you will want to screw the adjuster cap up or down to make the cycle frequency about 60 - 150 cycles per minute.
Experiment with the cycle frequency to find what delivers the most water for you. Lots of troubleshooting guides on the net to help you fine tune it. You may need to add or remove some weight off the bottom of the valve stem if it cycles too fast or slow and you don't have enough adjustment available by turning the upper valve cap. A little denial and error and you will find the sweet spot to get it running smoothly.
I have found that if it cycles too fast, it won't build up any pressure and the delivery flow is low or stops. If it cycles too slow, delivery flow is weaker and you use more feed water than necessary and you can drain the feed pipe if your supply water is limited. Mine seems to deliver the most water at a cycle rate of 150/minute.
Enjoy your gravity powered water pump, it's the closest thing you may ever see to perpetual motion... Hopefully you can see the movie of it working.
This is the best hobby project I have ever built from scrap parts and recycled junk. We use it every day all summer long and it facinates everyone who comes to visit us while saving real money!