The piston system when you pump air into the pressure chamber/resivour through a tube that is connected to the pump. The input of air to the pressure chamber/pump causes an increase in pressure, forceing water out through the nozzle. There is no valve between the nozzle and presusse chamber.
Varitions on the Piston System
Rotor Varient
The Pump is powered by a rotor that the user turns. Known to be used in Shield Blaster Guns.
Squirt Pistol Varient
The standard piston pump handle is replaced by a trigger. Nozzles in this varient are generally have small holes. As the name implies, this varient is commonly found in squirt pistols, but there are cases of this varient in soaker guns.