When using a plastic pump head with a cover for the first time, you must press the wrench several times before spraying the cleaning liquid. There are two reasons for this lag: When the liquid is initially drawn, the trigger must start a downward stroke, not an upward stroke (the piston is outside the hydraulic chamber).
When you pull the wrench back for the first time, the piston is pushed in, but no liquid is drawn out; there is only air in the hydraulic chamber. The piston slides out on its own to suck out the liquid from the reservoir. When the piston begins its first upward stroke, the pump begins to draw cleaning fluid from the reservoir.
But at the same time, it also sucked up the air in the plastic tube connecting the reservoir. Before you start spraying the cleaning fluid, you have to use the pump mechanism to remove the air. Therefore, you may need to move the wrench back and forth several times.
This simple pump head is designed as a reciprocating piston pump head, which is widely used. In addition to being able to press out water, air and other liquids, this plan can also be used to extract water and oil from the ground.
Even in our body, there is a reciprocating pump system: when your heart expands, it sucks low-pressure blood through a one-way valve; when your heart is shortened, it pushes high-pressure blood back into your body from another one-way valve. The same basic mechanism allows a general plastic pump head with a cover to work, and also allows us to live in this world!