The UK quattro Network
The UK's mobile Audi 5-cylinder mechanic - 07833 654800, 24x7
Specialist in turbos, quattros, electrics and non-starts

The Audi MB and RR Engine ur-quattro Braking System.

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

On the MB and RR Audi ur-quattros, a tandem pump provides both steering and brake assist. The rear part of the pump is a standard power steering pump fed by a translucent header tank. A small internal hole feeds a small amount of oil into the front section of the pump which uses two tiny pistons driven by an eccentric to supply high pressure hydraulic oil for the brake assist system. The smaller of the three hoses coming from the pump goes to the brake pressure accumulator which contains a built-in regulator.

The brake pressure accumulator is connected directly and permanently to the brake servo. The only useful sender in the system is a pressure switch mounted diagonally on the side of the brake servo. This is connected, as is the brake fluid level sender, to a single red warning light on the dashboard.

It is possible for the system to underperform by 30% without this being indicated by warning lights. 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 connectors off the brake servo and check that the contacts are closed. If not, replace the switch. This switch has a high incidence of "dead on arrival" and a new one that doesn't solve the problem should be checked in a known good car. If it works, the car must be taken to an Audi dealer for diagnosis with specialist equipment.
• 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 turn 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 probably needs to be replaced.
  • 4 - 10 pumps; check the system again every 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. Remove the drive belt and turn the pump by hand - there should be sudden stiff resistance twice per revolution as each of the pistons is operated by the eccentric. Once is enough to provide as least some assist, but is a clear indication that one of the pumping pistons has failed. Depending on the mode of failure (the pistons share a common output chamber and the efforts of the remaining one may be draining back through the defective one) the pump may not deliver adequate volume and pressure to operate the brake assist system properly. A freely turning pump is defective and needs immediate replacement or rebuilding.
• If the pedal goes hard while the vehicle is parked, 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 at the very top of the header tank by cutting through the clip with a hacksaw - do NOT try to open this clip with a screwdriver. Place the end of the 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.

Kerbside Motors Home Page and Contact Information

Contact Phil Payne by email or use mobile/SMS 07833 654800