The audible release of air comes exclusively from the pump hose, not from the suspension element. At this point, the valve is already closed, so the set pressure inside the suspension element remains fully retained. This brief hissing sound is technically normal and does not indicate any leakage.
Due to the approximately 2 m long steel-braided hose, a slight pressure drop can occur in the suspension element when connecting the pump, typically between 30 and 50 PSI. To minimize this pressure loss, proceed as follows:
First, connect the first stage of the valve connector to the suspension element to seal the system.
Then carefully pre-pressurize the hose to approximately the same pressure as the suspension element.
Finally, open the second stage to allow airflow and enable pressure equalization.
This way, the pressure in the suspension element remains nearly constant during connection.
There's an explanatory video here!