Two factors affect the flow rate:

  1. The distance from the reservoir.  “Head” which is measured in distance from the filter to the top of water level in the reservoir can be converted to pressure, or “PSI” (pounds / in²).  The conversion is 1 psi = 27.6 inches of water.  Using this conversion you can see that it would take 6.9 feet of tubing to reach 3 psi which meets the maximum performance for the Red Line filter.
  2. Air lock.  It takes a blast of pressure to expel the trapped air and replace it with water which will restore the full surface area of the filter to water flow and allow the filter to perform as intended.  Usually this requires a squeeze bulb or squeezing the bag to expel the air.