My reverse osmosis filter is having some trouble. I am not getting any water flow from it whatsoever.

I changed the small flavor filter, air tank, flow regulator on the waste line, all three initial stage filters, and the reverse osmosis element. Basically the only thing that I did not replace was a short, silver cylinder that has 4 tubes on it. I don't know what that is. I know that the water line to the filter is good, and I can even hear a trickle from the waste line, but I get no water to the taps.

Anyone have any ideas what might be the problem? What other information do you need? And what pressure should I keep the tank at? I've been playing around with various pressures, but nothing seems to work.