Water runs down the drain even though water storage tank is full.

This can be caused by faulty 4-way valve and as a result the valve does not shut off the water to the RO membrane.

Also, check if there is sufficient water pressure in your pipes and system (minimum of 2.8 bar or 40.6 PSI). If the pressure is lower, install booster pressure pump  AFXPOMP or AFXPOMP-4.

It is also possible that your system's 4-way valve may be damaged by dirt or sediment particles that got into the system during installation. In such cases, 4-way valve needs to be disassembled and cleaned up. If the problem persists, replace the valve with a new one.

Another reason could be damaged seal of one of membranes forming the 4-way valve.

And finally,  4-way valve can be clogged by large amounts of ions of calcium, magnesium and iron. In such cases, 4-way valve needs to be disassembled and cleaned up. If the problem persists, replace the valve with a new one.