Wet valve with soapy water. If an air bubble forms, screw the valve -3- firmly with the dust cap -2- turned upside down.
Note: The valve may only be tightened with the metal cap -2-. This cap is located on the side of the fuel tank. 1 - rubber protective cap; 4 - valve body.
Check the valve again. If air bubbles still form and the valve cannot be tightened any further, it must be replaced (workshop work).
Be sure to reattach the dust cap.