At Citroën riders are the "red system" and "green system" known phenomena. The different hydraulic fluids and the associated systems are not compatible.
Now that Audi's are also classics and / or youngtimers, a color problem regularly occurs. Audi enthusiasts should pay close attention to whether the right fluid comes in their steering and brake servo system. And the right fluid must be a GREEN hydraulic oil instead of a normal RED ATF.
The cap on the liquid container indicates the color of the required liquid (see photo)
The green juice is thinner. The red ATF thereby tends to be aggressive towards the seals in the sturgeon house. If your 'green' Audi has red ATF, the system should be flushed with at least one liter of green liquid while the engine is turned several times from one extreme position to the other.