Reverse Osmosis Tank Lost Pressure? How to Check It and When to Replace
Oct 03, 2026The short version: the storage tank under your sink is what pushes reverse osmosis water up to the faucet. Inside is a rubber bladder with a small air charge behind it, about 7 psi on most tanks when the tank is empty. When that air leaks down or the bladder fails, you get a cup or so of water and then a trickle, even though the tank still feels heavy. Low air can be topped up with a hand pump. A failed bladder means a new tank, and on an older system it is often the point where replacing the whole unit makes more sense.
What the tank does
A reverse osmosis membrane makes water slowly, a few ounces a minute. Nobody wants to wait two minutes for a glass of water, so the system fills a small tank in the background and you draw from that.
The tank has two sides. One holds water. The other holds air, separated by a rubber bladder. As water fills the tank it squeezes the air, and that squeezed air is what pushes the water back out when you open the faucet. There is no pump on a standard tank system. The air charge does all the work.
A typical under-sink tank is sold as 3 to 4 gallons, but it delivers closer to 2 or 3, because the air side takes up part of the space. After it runs empty, expect two to four hours to refill.
Signs the tank is the problem
| What you see | What it usually means |
|---|---|
| A cup or two comes out strong, then it drops to a trickle. The tank still feels heavy. | Low air charge, or a failed bladder. The water is in there but nothing is pushing it out. |
| Slow from the first second, and the tank feels light. | The tank isn’t filling. Look at the filters, the membrane and the feed valve before the tank. |
| Water comes out of the air valve when you press the pin. | The bladder has ruptured. The tank needs to be replaced. |
| You added air and it worked, then it faded again within a few months. | The air valve or bladder is leaking. Replace the tank. |
| Flow is fine at the sink but weak at the fridge or ice maker. | Normal for a tank system on a long line. Tank pressure drops as it empties. |
If the tank feels light and the faucet is slow, the problem is upstream. Our guide to cloudy, slow or off-tasting RO water covers filters, membranes and the drain line.
How to check the air charge
- Close the cold-water feed valve to the RO so no new water is being made.
- Open the RO faucet and let it run until it stops.
- Lift the tank. An empty tank is light enough to pick up with one hand. If it is still heavy, water is trapped inside with no air pushing it out.
- Find the air valve. It looks like a tire valve, usually under a small plastic cap on the side or bottom of the tank.
- Check it with a low-pressure gauge. Most tanks call for about 7 psi when empty. The label on your tank has the exact number.
- If it is low, add air with a bicycle or hand pump while the faucet is open. The air will push the trapped water out. Keep going until the water stops and the gauge reads the number on the label.
- Close the faucet, open the feed valve and give it a few hours to refill.
Go slowly and use a hand pump. A shop compressor can overfill a small tank in a second and tear the bladder. And only check the pressure with the tank empty. A reading taken on a full tank tells you the water pressure, which is a different number.
Repair, new tank or new system
| Situation | What we’d usually do |
|---|---|
| Air was low, tank holds it after a top-up | Nothing else. Check it again at the next filter change. |
| Bladder failed, system is newer and parts are easy to get | Replace the tank and sanitize the system while it’s apart. |
| Bladder failed, system is old, membrane and filters are overdue, or parts are hard to find | Replace the whole system. A new tank on a worn-out unit is money spent twice. |
| Cabinet is tight, or the fridge line has always been slow | Consider a tankless reverse osmosis system. It has a built-in pump and no storage tank, so there is no bladder to fail. It does need an outlet under the sink. |
From a job this week in Bradenton
This week in Bradenton we took out an older under-sink reverse osmosis system with a failed storage tank. The whole unit came out and a new Puronics Micromax 7000 went in. You can see recent work on our Bradenton page.
Why tanks fail sooner here
Under-sink cabinets in Florida get warm and humid, and rubber ages faster in heat. Tanks that sit in a garage or outside age faster still. Many tanks last somewhere around 5 to 10 years. If yours is in that range and the faucet has gone weak, the tank is the first thing to check.
Common questions
Why does my RO faucet run strong for a second and then trickle?
The storage tank has lost its air charge or the bladder inside has failed. There is water in the tank, but nothing is pushing it out.
What air pressure should a reverse osmosis tank have?
Most under-sink tanks call for about 7 psi with the tank completely empty. Check the label on your tank for the exact figure.
Can I add air to my RO tank myself?
Yes. Close the feed valve, open the faucet, and add air with a hand or bicycle pump until the water stops and the gauge reads the number on the tank label. Don’t use a shop compressor.
How do I know if the RO tank bladder is ruptured?
Press the pin on the air valve. If water comes out, the bladder has failed and the tank needs to be replaced.
How long does a reverse osmosis tank last?
Many last around 5 to 10 years. Heat shortens that, so tanks in a Florida garage or a warm cabinet tend to go sooner.
How long does an RO tank take to refill?
Usually two to four hours for a standard under-sink system, depending on the membrane size and your water pressure.
Is a tankless RO better than a tank system?
It depends on the home. Tankless systems save cabinet space and keep steady pressure for a fridge line, but they need power under the sink. Tank systems are simpler and work without electricity.
RO faucet down to a trickle? We’ll take a look and tell you plainly whether it needs air, a tank or a new system. Call or text 941-266-8991.
Related: RO water cloudy, slow or tastes off · Tankless reverse osmosis · RO vs. pitcher and fridge filters · Does reverse osmosis reduce PFAS? · Softener vs. RO: do you need both?
Sources: RO storage tank manufacturer pre-charge specifications; US Water Systems RO tank service guidance; Hydrology Water Solutions service records.
