In an under-sink reverse osmosis system, the storage tank holds treated water until you open the faucet. Inside, a flexible bladder separates the stored water from a pocket of air. That air pressure pushes the water out to the faucet. When the air charge is right, the faucet runs with steady flow. When it is wrong, the tank still holds water, but you lose flow or get sputtering.
What the air charge does
The tank's air valve, which looks like a small tire valve, sets the pressure on the air side of the bladder. The manufacturer's manual states the correct charge. Read it and write it on the tank or in your maintenance notes so you are not guessing later.
How to check it
Close the feed valve to the system. Open the faucet and let the tank drain completely, then close the faucet. With the water side empty, use a tire gauge on the air valve. If the reading is well below the specified charge, add air with a bicycle pump or similar, then recheck. Do not check the air charge while the tank still holds water, because the reading will be misleading.
Signs the tank needs attention
Weak or sputtering flow at the faucet can point to a low air charge, a clogged restrictor, or a failing membrane upstream, so check the air side first because it is the cheapest to test. If the tank feels heavy with water when empty, or the air valve releases water, the bladder may have ruptured. A ruptured bladder means the tank needs replacing, not just more air.
Keeping the fittings sound
The tank's inlet and outlet fittings carry treated water. Check them whenever you change filters. Look for slow drips, mineral crust, or a loose collar. Tighten a loose fitting gently, and replace the tubing if it has hardened or cracked at the ends.
Building it into routine service
Add the air check to the filter service visit so it happens on a schedule rather than after a complaint. Note the reading, the date you did it, and any changes. Over time, a slow drop in air charge tells you the bladder is aging, which is useful when you plan the next replacement.