This problem may have several causes. If the mileage of the car is more than 400,000 kilometers, then most likely the is failure in the air suspension- there is a leak inside. Although the air suspension at the VW Touareg can fully work up to 600,000 kilometers. But most often there are problems with the connection of the fastening support «hat» with the air suspension balloon itself — air leaks appear in 80% – 90% of cases.
In this situation, it is necessary to remove and disassemble the cylinder, clean the seating surfaces, replace the gasket and to seal. Since the surfaces are made of aluminum, they oxidize over period of time, so it is necessary to clean and replace the gasket.