The 1990 2309cc model has a hydraulic booster instead of a vacuum booster. The system uses pressure from the power steering system.
The gas-filled accumulator maintains a pressure reserve and includes a relief valve and a one-way valve. Liquid under pressure is received from the accumulator into the booster unit.
The hydraulic booster unit is located between the brake pedal and the master cylinder. Each component operates on a completely separate hydraulic line and is provided with separate feed tanks.
The procedure for removing and installing the pump is given in Section 10.
[The full version is posted on the resource: AudiManual]