If the check valve isn't original equipment then take it out. I don't know what size tank you have but it can take many hours to fill. The less water in the tank the lower the pressure.
@Santa - It's been a few years, but I think that a check valve might have been installed originally. The last time that replaced the filters a few years back, I believe that it fell out - and I had no idea where it came from. At the time, I didn't have the same diagram that I attached to this posting - which now shows where a check valve is inserted.
You need to have your tank to atmospheric pressure (14.7 psi) - drain the tank completely, then depress valve for a minute, the static pressure in the room will fill the area between the bladder and tank, then when the tank (bladder) is full - the atmospheric pressure will act as a cushion to dispense water. The unit will take up to 24 hours to fill the tank, so a gallon or so will be available. If you require more than this, fill a container and refrigerate and repeat process. while the unit is processing, you will hear a slow dribble of water down the drain
As a rule, the check valve is built into the membrane housing - be certain that the membrane is installed in the right direction
@kdog - I drained my tank (it looks like it is about 2-3 gallons), and then found an accurate low pressure gauge (1-20 psi) at the auto store. I pressurized the tank bladder to 5 psi when the tank was empty.
I'm 99.9% sure that I have the RO membrane installed in the right direction - it seems like there was only one logical way to put it in the housing.
Your comment on the check valve being built into the membrane housing makes a lot of sense, and makes me wonder if I have 2 check valves in the loop now. If I take the system apart, will I be able to see if a check valve is in the housing?
While it seems like the system is working pretty much OK with the (possibly extra) check valve installed after the RO filter, I still am not sure that the system is working the way it is supposed to. This is the first RO system I have owned.
I know that there is a huge difference in water flow rate if I remove the check valve that I inserted. My RO membrane is a 24 gallon-per-day unit. With the tank empty and the "new" check valve installed, the flow rate from the RO is not much above a trickle - pretty much what I would see if I barely turned on a normal faucet and see a stream. If I remove the "new" check valve and the tank is empty, the flow rate pretty steady at about 75% of what I would see with RO the tank full.
The first (slow trickle) flow rate might actually be closer to a gallon per hour, though. I just don't have enough experience to know if what I have now is the proper operation.