The airbags on my low-mileage 212 were leaking. Weak in cold temperatures, but fell uniformly on both sides. Never dropped again after replacement.
It can be more costly to experiment with the diagnosis technique when you're also paying for labor (multiple visits), therefore I'm assuming BlackC55's proposal is based on their extensive experience. If the air springs aren't going to last very long anyhow, and you plan on keeping the car for a while, replacing the pipes and valve block might be less expensive and less of a hassle than completing the job twice.