Why use PET spray hose twice more?

Began to extract liquid, pull a gun to start a downward stroke, rather than upward stroke ( The piston in the hydraulic chamber) 。 When back wrench for the first time, the piston is pushed down inside, but there is no liquid out; Only the air in the hydraulic chamber. Piston sliding out himself, to suck out the liquid from the receiver.
when PET piston spray hose start up stroke for the first time, the pump start drain cleaning fluid from the receiver. But at the same time, it will also be connected to the receiver in plastic pipe air sucked up together. Prior to spray cleaning fluid when you start using the mechanism of the pump will be to get rid of the air. Therefore, may need to throw a wrench a few times back and forth.
this simple design is called the reciprocating piston pump, a wide range of applications. In addition to the pressure of the water, air and other liquid, this design can also be used to extract oil and water from underground.
in the human body, even also has a reciprocating pump system: heart expands, through a one-way valve inhaling low blood; Cardiac contraction, the high pressure of blood from another one-way valve pressure back to your body. The same basic mechanism, make an ordinary PET spray hose, also let the human survival in this world!