2. The remaining checks require a pressure gauge, so it is best to have this work done by an Audi dealer. However, if a pressure gauge is available, the procedure diagram is given in the following paragraphs.
3. Relieve all pressure from the feedback hydraulic system by depressing the brake pedal with the engine off approximately twenty times.
4. Disconnect the wiring from the differential pressure indicator, then unscrew the switch, remove the washer. Connect the pressure gauge.
5. Run the engine at idle speed until the pressure reaches approximately 140 bar.
6. Turn off the ignition, then pump the brake pedal, watching the pressure gauge. The pressure should first drop slowly, and then suddenly drop to zero. The pressure before dropping to zero is the accumulator pressure, and if it is less than 30 bar, the accumulator must be replaced.
7. If the pressure specified in point 5 is not achieved, first check the tension of the pump drive belt, using Chapter 2 of Section 10.
8. If the drive belt tension is correct, check the pump performance by connecting the special hose and the limiter (VAG 1354/1) to the pump pressure outlet. With the engine idling, the pumping rate should be at least 0.3 liters per minute. If not, replace the pump.
9. Remove the special hose and restrictor, install the pump outlet pipe.
10. The accumulator includes a relief valve and a one-way valve. If the pump performance is normal but the pressure specified in point 5 cannot be reached, the relief valve is faulty and the accumulator must be replaced.
11. To check the one-way valve, run the engine at idle speed until maximum pressure is reached. Switch off the ignition, pump the brake pedal until the pressure is approximately 135 bar. Check that the pressure does not fall below 130 bar for five minutes. If the pressure falls below 130 bar, the valve is faulty and the accumulator must be replaced.
12. At the end of the check, remove the pressure gauge, install the differential pressure indicator together with a new washer, if necessary. Reconnect the wiring.
[The text is based on materials from the website «audimanual»]