Diagnosis
That’s a stuck Automatic Shut-Off (ASO) valve. Its job is to shut off feed water to the membrane when the tank is full. If it’s stuck open, water flows through the membrane and straight to drain indefinitely.
How to Fix It
Locate the ASO valve (the black rectangular one with four ports). Gently tap its sides with the handle of a screwdriver to try and free up the internal piston. If that doesn’t work, you’ll need to replace it. To confirm it’s stuck, shut off the tank valve. If the drain flow STOPS after a minute, the ASO is likely okay and the check valve in the tank might be bad. If the drain flow continues with the tank valve closed, the ASO is definitely faulty.
If That Didn't Work
The check valve (a small one-way valve) on the tank valve assembly could be leaking, letting pressure from the tank back into the ASO valve, tricking it into thinking the tank is empty. This is less common but possible.
Heads Up
A constantly draining system wastes a ton of water and wears out your membrane fast. Don’t let this go for more than a day or two. It’s also a sign your feed water pressure might be too low (below 40 PSI) for the ASO to function properly.