Use the hose and the nozzle in order to expel all of the water from the tank. Allow the compressed air to escape through the nozzle also. Unscrew the filler plug for two turns counterclockwise. Do not turn the filler plug more than two turns until the pressure is relieved. There is a cross-drilled hole in the filler plug that will allow the pressure in the tank to be relieved. Make sure that the pressure is completely relieved before you remove the filler plug.
Make sure that the ball valve is shut off. Fill the tank with water. The total volume of the water tank is 33.9 L (8.95 US gal). The tank will hold 25.5 L (6.74 US gal) of water and 8.48 L (2.21 US gal) of air. Install the filler plug and pressurize the tank.
Note: If the outside temperature is below freezing, use a propylene glycol based antifreeze in the water tank. The antifreeze should be a non-toxic antifreeze that is used in recreational vehicles.
When the tank is full, reinstall the filler plug.
Connect an air hose to the valve that is located between the shut off valve and the pressure gauge. Pressurize the tank to 345 kPa (50 psi) according to the pressure gauge.
Turn the shut off valve to the ON position. Expel the water from the nozzle. If the discharge is not strong, repeat the purging process. If there is still a problem, remove the hose and the lines and check for an obstruction.
Once the purging process is complete, fill the tank and pressurize the tank.