A reverse osmosis storage tank works like a small spring. A rubber bladder holds the treated water on one side, and a pocket of air on the other side pushes that water out to the faucet when you open it. When the air charge drops, the tank holds less usable water, the faucet slows or sputters, and the unit may start and stop more often than it should. The change builds slowly, so many owners only notice it after the faucet has been weak for weeks.
What it looks like at the tap
Watch for a faucet that spits or coughs air at the start of a draw, a tank that refills and seems to stop sooner than it used to, or a thin stream that runs out before a glass is full. These symptoms look like a clogged filter, and owners often change cartridges that did not need changing. Before you spend money on a cartridge, check the tank.
Check the air charge with the tank empty
A pressure reading only means something when there is no water in the tank. Close the feed valve to the RO unit, open the treated-water faucet, and let the tank drain until the flow stops. Then find the air valve on the tank, which looks like a tire valve, remove its cap, and press a tire gauge onto it. Compare the reading with the setting printed on the tank or in its manual. Do not borrow a target number from another tank, because tanks are set differently.
Before you loosen any fitting, confirm that the feed is closed and that no water is still under pressure. If you are not comfortable working on the plumbing, ask your installer or provider to do this check at the next visit.
Read the result
If the gauge reads at or near the printed setting, the air side is fine, and the slow flow probably has another cause, such as a restricted filter or a valve that is not fully open. If the reading is well below the setting, the tank has lost air. Add air with a bicycle or tire pump up to the printed setting, then check again after a day. If the charge falls back within a few days, the air valve may be leaking or the bladder may no longer be sealing.
Watch what comes out when you press the gauge. If water sprays from the air valve, the bladder has failed and the tank is waterlogged. Adding air will not fix that, so plan on replacing the tank.
Questions to put to your provider
Ask whether the storage tank is sold and warranted separately from the membrane and cartridges, and whether the warranty covers a failed bladder. Ask what pressure was set on the tank at installation and where that number is written in your paperwork. Ask how the charge should be checked at each service visit, so you can repeat the check between visits. If a provider cannot answer these questions clearly, that is worth knowing before you need the next service call.
Keep a simple record
Write down when you checked the charge, the reading you got, and the setting printed on the tank. A slow drop across several checks tells you more than one reading does, and it gives a technician something concrete to work from when you call.