The storage tank on a reverse osmosis system is a pressure vessel with a rubber bladder inside and a cushion of compressed air behind it. That air charge is what pushes water out to the faucet, and it leaks away slowly over years. When it does, the symptoms look like a failed membrane and the fix takes ten minutes.
How the tank actually works
Water fills the bladder against the air cushion, compressing it, and the compressed air pushes the water back out when the faucet is opened. The air charge is set with the tank empty, and it has to be low enough that the membrane can fill the tank against it and high enough to deliver a usable flow. Too high and the system produces almost nothing because the membrane cannot push against the cushion. Too low and the tank holds water it cannot expel.
The symptoms of a wrong charge
A trickle from the faucet that stops almost immediately, with the tank feeling heavy, means the charge has bled away and the water is sitting in a bladder with nothing to push it. A tank that never fills, with the system running to drain constantly, often means the charge is too high. A faucet that spits air is a ruptured bladder letting air into the water side. Owners commonly read the first as a failed membrane and replace the expensive part while the tank sits uncorrected.
Checking and setting it
Close the tank valve and open the faucet until the tank is completely empty, which matters because a charge measured with water in the tank is meaningless. Find the air valve under the cap on the tank, usually a standard tire valve, and read it with a low pressure gauge, since an automotive gauge is too coarse to be useful here. Adjust with a hand pump to the manufacturer's figure, add in small increments, and recheck. Then reopen the valve and let the system refill.
When the tank itself is finished
If the charge will not hold for more than a few weeks, the valve is leaking or the bladder has failed and the tank is a replacement rather than a repair. Air coming out of the faucet, or water escaping from the air valve when it is depressed, both confirm a ruptured bladder. While the system is apart, check the automatic shutoff valve and the permeate check valve, since those cause the other production faults, and sanitize the tank before recommissioning. Our reverse osmosis guide covers the full service.
Questions people ask about reverse osmosis water tank pressure
Why does my RO faucet only give a trickle?
Usually a lost air charge in the storage tank. The water is in the bladder with nothing behind it to push it out.
How do I check the tank pressure?
Empty the tank completely first, then read the air valve with a low pressure gauge. A reading taken with water in the tank means nothing.
Water comes out of the air valve, what does that mean?
The bladder has ruptured. The tank is a replacement rather than a repair at that point.
The system runs to drain and never fills, why?
Often the air charge is set too high for the membrane to fill against, though a failed automatic shutoff valve does the same thing.