Low Pressure Gauge in one pound increments to check your R.O. Storage Tank ( proper pressure is 7 - 8 lbs. ).
The Problem: Low Volume and Poor Pressure
Little to No air pressure will result in very little water being pushed out of the holding tank. You will have a full and heavy holding tank, but with out the air pressure, you'll never get much water out of the tank. Too much air pressure ( say over 10 psi on a small tank ) will result in good water flow but will severely limit the amount of water that can be stored in the holding tank. So, it is recommended to put 8 psi of air in your under the sink reverse osmosis holding tank. If you get water coming out of the air valve, then it is time to purchase a new holding tank.

To Check:
With all possible water drained from the holding tank, you want to get an air pressure reading of 8 PSI of air pressure. There is a "tire" valve on your holding tank somewhere ( usually the bottom ). You really should use a special low pressure air gauge. One that shows the air pressure in one pound increments starting with one pound. The average tire gauge starts at 5 or 10 lbs. psi.