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The WR ur-quattro ATF (Dexron) Braking System

The hydraulic brake assist system is a critical safety feature and must be in working order.

On the 1980-1987 cars, a single vane-type pump provides hydraulic oil under pressure for both steering and brake assist. This feed is split in the brake pressure regulator which is mounted underneath the brake pressure accumulator (the "bomb"). Up to 0.7 litres of oil per minute can be diverted to maintain brake assist pressure - the rest recirculates through the power steering gear.

The brake pressure accumulator is connected directly and permanently to the brake servo. There may be one or two senders in the system depending on the age of the vehicle. The one mounted high on the side of the brake pressure regulator is operated by the pressure in the accumulator - the one lower down is operated by the supply pressure from the pump. Both senders are normally connected to the voice synthesiser. The earlier single-sender brake pressure regulator was superseded by the two-sender version - early cars may thus have two senders with only one connected - this should be the upper one.

It is possible for each component of the system to underperform by 30% without this being indicated by any warning light or the speech sythesiser, even assuming the senders are working to specification. A complete system check is advisable at least every 10,000 miles.

• Check that the hydraulic fluid level is at least above the minimum mark and there are no leaks in the system.
• Pump the brake pedal hard at least thirty times to discharge the accumulator.
• Pull the connector off the upper pressure switch on the brake pressure regulator and check that the contacts are closed. If not, replace the switch. If this does not solve the problem, the car should be taken to an Audi dealer because highly specialist measuring equipment is required.
• Start the engine and check that the brake servo switch opens. If not, the hydraulic pump is probably defective.
• Run the engine for a few minutes and then switch it off.
• Start pressing the brake pedal, looking for the brake servo switch to close again - the pedal will also go hard at this point.
  • 1 - 3 pumps; the system is potentially dangerous and the brake accumulator almost certainly needs to be replaced.
  • 4 - 10 pumps; check the system again in 5,000 miles. Replace the brake pressure accumulator if planning to drive in mountainous country
  • 11 - 30 pumps; the system is in order
• If the pedal remains hard and the switch remains closed, the hydraulic pump has probably failed. Again, highly specialist measuring equipment is required.
• If the pedal goes hard overnight, the most likely cause is servo leakdown. Very serious servo problems can also cause the system not to pressurise - the brake servo switch remains closed and the pedal remains hard. To check for this, remove the return hose from the servo to the reservoir at the reservoir end. It is necessary to block off the reservoir attachment - this can be done by slipping on a length of hose and either appling a brake pipe clamp or lifting the other end above the reservoir level. Place the end of the servo return hose in a small jar, run the engine for a few minutes and observe - only a very few drops should come out of the hose. More tham 10ml overnight indicates a leaking servo.

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