The way how storage tank work is by means of air pressure and water pressure. The storage tank is divided in two with a rubber diaphragm.
Bottom side of the RO Storage Tank Working:
The underside of the reverse osmosis tank in filled with normal air. You will notice an air valve on the side or on some tanks it is at the bottom. If you push the air valve in you will notice that air is coming out.
Top side of the storage tank working:
The top part of the storage tank is been filled with water from the reverse osmosis system. The top section is lined out with a thin plastic lining to keep the water free from rust or a steel taste. The moment the water seeps through the side of the diaphragm and the plastic lining. You will find that the water from the faucet starts to flow weaker until it is just a very thin stream of water that comes out. Sometimes the system will only give you a glass full of water. That is a sign that the air pressure in the bottom section of the tank is depleted.
To check the air pressure in the storage tank:
Close the water inlet valve from the house to the reverse osmosis system. Open the faucet and let all the water flow out. Close the tap on-top of the storage tank. Take out the tank and unscrew the pipe to the tank valve. Take the storage tank to the outside of the house and open the tap of the tank. Let the water flow out. If the water flow stop. Take a compressor or a pump and pump some air in the tank at the air valve. Do not put too much pressure in the tank. If the water stops flowing out. Measure the tank air pressure. Recommended air pressure if the tank is empty and the tap is open must be 0.8 bar or 80 kpa. There is no other way to check the tanks air pressure. If there is still water in the tank it will influence the tanks air pressure.
It is very important to test the tanks air pressure when the tank have no water in. I have mentioned it before but if you do not do as I say you will damage the diaphragm and then you must buy a new tank.