A reverse osmosis system normally sends some water to the drain while it is producing filtered water. The warning sign is drain flow that continues long after the storage tank should be full and nobody has used the drinking water faucet.
Do not diagnose the problem from drain noise alone. An air gap faucet can make normal production unusually easy to hear, and a slow trickle may be difficult to distinguish from water moving through another fixture. First prove that the flow belongs to the reverse osmosis system, then find out whether the system is producing continuously or failing to recognize that the tank is full.
Confirm that the drain flow is actually continuous
Choose a period when nobody needs filtered water. Close the drinking water faucet and stop any refrigerator or ice maker connected to the system from drawing water. Give the storage tank time to refill. The required time depends on the membrane, incoming pressure, water temperature, tank condition and recent water use, so do not use a short observation as proof of failure.
Locate the small drain tube from the reverse osmosis system. Depending on the installation, it may connect through an air gap faucet or directly to a drain adapter under the sink. Listen and feel for flow without loosening the drain connection. If the drain line remains active after the tank has had ample time to fill, close the system's feed water valve. The drain flow should stop. If it does not, make sure you have identified the correct tube and valve before proceeding.
Record what you observe: whether the storage tank feels full, whether the faucet delivers a strong stream, whether the drain flow is steady or intermittent, and whether closing the feed valve stops it. These details help separate a production problem from ordinary drain noise.
Check whether the storage tank can create shutoff pressure
Most under-sink reverse osmosis systems stop producing when pressure in the storage tank rises high enough to operate an automatic shutoff valve. If the tank cannot build or hold that pressure, the system may continue producing and draining.
With the feed water open and the drinking faucet closed, gently lift the tank if it is accessible. A tank that feels heavy may contain water, but weight alone does not prove that its air charge and internal bladder are working correctly. Open the drinking faucet and watch the flow. A brief strong stream followed by a rapid decline can point toward a tank that is not delivering its stored water properly. A tank that never becomes heavy may indicate that little product water is reaching it.
Do not add air to a full tank based on a guess. Tank air pressure is checked with the water side empty and isolated according to the tank or system instructions. Measuring a water-filled tank produces a misleading result. If water comes out of the tank's air valve, the internal bladder has likely failed and the tank generally needs replacement rather than adjustment.
Make sure product water is not escaping somewhere else
A refrigerator, ice maker or second faucet connected to the reverse osmosis line can draw water without anyone noticing. A leaking fixture valve can prevent the tank from reaching shutoff pressure even when no visible faucet is open.
Trace every tube leaving the filtered water side of the system. Temporarily close an installed branch valve if the system provides one, then see whether the tank fills and the drain flow stops. Do not kink tubing or cap a line with an improvised plug. If there is no proper isolation valve, leave that test to a service technician.
Also inspect the drinking faucet for a slow drip and the cabinet for damp tubing connections. A small product water leak matters because the system may repeatedly replace the lost water and send reject water to the drain while doing so.
Separate low production from a failed shutoff
A restricted prefilter, aging membrane, cold feed water or inadequate incoming pressure can slow production. A system producing very slowly may appear never to finish filling its tank. That is different from a system that fills the tank but continues draining because the automatic shutoff valve, check valve or related tubing is not working correctly.
Run the tank empty into a clean measured container only if you need a baseline and can follow the system instructions. Then close the faucet and observe how the system behaves during refill. Useful questions are simple: Does water reach the tank? Does faucet performance improve after a full refill? Does drain flow stop once the tank is pressurized? Avoid judging the membrane by drain flow alone because the same symptom can come from several components.
The check valve is another possible failure point. It is intended to keep pressurized tank water from pushing backward through the membrane. If it leaks internally, tank pressure may bleed away and repeatedly restart production. Testing it usually requires isolating tubing and knowing the system's flow path, so it is a reasonable stopping point for a homeowner who does not already service reverse osmosis equipment.
Inspect for installation conditions that prevent shutoff
Look for sharply bent tubing, incorrectly routed connections and valves that are only partly open. Compare the tube locations with the system diagram, especially if the problem began after filters, a membrane, a tank or tubing were replaced. A tube installed in the wrong port can create convincing symptoms while making part replacement useless.
Check incoming pressure using the method specified by the equipment manufacturer if you have the correct gauge and connection. Do not rely on how forceful a nearby faucet feels. Household pressure can change when other fixtures run, and a pressure reducing valve, booster pump or well pressure cycle may affect the reverse osmosis system differently.
If the installation uses a permeate pump, booster pump or electronic shutoff control, include it in the diagnosis. Ask the technician to explain what signal or pressure should stop production and to demonstrate that the stop actually occurs.
Know when to stop the homeowner checks
Close the feed water valve and arrange service if drain flow is continuous, the cabinet is wet, tubing is damaged, a connection will not hold or you cannot identify the correct shutoff. Keep the reverse osmosis faucet closed while the system is isolated. If a refrigerator depends on that water line, turn off its ice making function or follow its instructions for loss of water supply.
Do not start replacing the membrane, tank and automatic shutoff valve one after another. That approach can cost more while leaving the actual pressure or plumbing problem untouched.
Ask for a diagnosis you can verify
Before approving a repair, ask the provider to identify which condition prevents shutoff. The explanation should connect the symptom to a test, such as measured feed pressure, verified tank air charge with the tank empty, confirmed reverse flow through a check valve, restricted production or failure of the automatic shutoff valve.
Ask what parts will be replaced, why each part is needed and how the system will be tested afterward. A useful completion test includes refilling the tank, confirming usable faucet flow, checking every disturbed connection for leaks and demonstrating that drain flow stops when the tank reaches shutoff pressure.
If you need to choose a service company, compare whether providers diagnose the complete water and pressure path rather than quoting a membrane or tank from the symptom alone. The site's water treatment provider ratings can help you identify companies to question. The goal is not merely a quieter drain. It is a system that produces, stores and stops predictably.