Ag Spray sprayers are designed to spray a large variety of Agricultural and Lawn & Garden chemicals diluted in water.  The pumps do not pressurize the tank, but actually draw the solution from the tank and pressurize the sprayer's plumbing system.

Piston and check valves inside the pump create the suction to draw the water from the tank and then force the water through the discharge port of the pump.  On basic spot sprayers, the only pressurized plumbing is the 15 foot of hose and the handgun.

Shurflo pumps used on Ag Spray sprayers are "demand" pumps that only run when there is an open outlet for the spray.  When the lever on the handgun is squeezed to spray, the pressure in the plumbing drops and the pump will run.  When the lever is released, a pressure switch located on the pump head senses the pressure increase and shuts the pump off.  The pressure switches are preset at the factory and are different for the different sized pumps.