The suction is created by the operation of a pumping unit inside the dispenser. The pump creates a vacuum. The vacuum pulls product from the storage tank, through the piping.
Early gasoline stations all operated through the use of suction systems and some such systems are still in use, particularly at private fueling facilities. In most modern retail stations, however, suction systems have been replaced by remote pumping systems in which the pumping unit is located inside the storage tank. In a remote system, the liquid product is “pushed” from the storage tank to the dispenser.