Most point-of-use reverse osmosis systems hold treated water in a small pressurized storage tank. Inside the tank, a rubber bladder holds air at a set pre-charge, and the water sits on the other side of the bladder. When the air pressure drops, the tank holds less usable water and pushes it out less firmly, so the faucet slows down. The problem develops slowly, which is why it is easy to miss and why it often gets blamed on something else.
The early signs
The first sign is usually a change in how the faucet behaves at the end of a draw. The water may slow down sooner than it used to, sputter or spit air as the tank empties, or come out weaker than it did after the system was first installed. Some households notice the system refilling its tank more often than before, or a faucet that seems to run slowly even when the tank looks full.
These symptoms can also come from a clogged filter, a restricted faucet, or low feed water pressure to the system. That overlap is why the tank check below is worth doing first. It takes a few minutes and costs nothing beyond a tire gauge.
How to check the tank
Shut off the feed water valve to the reverse osmosis system, then open the RO faucet and let the storage tank drain until the water stops. With the tank empty, find the air fitting on the tank. It is usually a tire-style valve under a small cap on the side or bottom. Press a standard tire gauge onto it and read the pressure.
Compare that reading with the pre-charge printed on the tank label or listed in the manufacturer's manual for your model. Use the number for your tank, not a figure from a general website, because pre-charge settings vary. A reading well below the stated value means the tank has lost air. A reading at or near the target means the tank is probably not the cause, and you should look next at the filters, the faucet, and the feed pressure.
Check the feed pressure at the RO inlet with a gauge if you can. Low feed pressure produces many of the same symptoms, and no tank adjustment will fix it.
What to do with the result
If the tank is low, recharge it to the stated value using a tire pump or another regulated air source, then close the valves and check the reading again after a day or two. Recharge slowly and watch the gauge, because overfilling the bladder can damage it. If the air leaks out again quickly, the bladder is likely ruptured, and the tank needs replacement rather than another recharge.
Keep a short written record of each reading and what you did. If you later call a provider, that record shows exactly what you already ruled out, which shortens the conversation and keeps the visit focused on the real fault.
Questions to ask before you pay for a repair
Ask what pre-charge your specific tank requires and where that number is printed or documented. Ask whether the tank is sold separately or only as part of a service call, and whether a replacement tank matches your existing fittings. Ask whether the feed pressure was measured, not assumed. A provider who checks the tank, the feed pressure, and the filters before recommending a new membrane is doing the work you need.