Problem with reverse osmosis filter

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.

The "thing" with 4 tubes on it may be the pressure regulator, which determines whether the unit has to produce water or not. It could be defective, or you could have too much air in the tank so the regulator thinks it is full already. If you get a trickle down the drain, it should be making water.

While I look into getting a new regulator, can you tell me what is a good pressure to keep the tank at?