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Bleeding the brakes on an Audi ur-quattro

Procedure:

• Use DoT 4 specification brake fluid.
• Before starting any work, ensure that all brake bleed nipples can be moved.
• If at all possible, use proper flare nut spanners on each bleed nipple.
  • 11mm on master cylinder - occasionally 10mm.
  • 9mm on brake proportioning valve (below master cylinder) - flare nut socket useful as space is limited
  • 11mm on each caliper - sometimes 7mm.
• Optionally remove the brake light bulbs from the rear light clusters - the current drain is considerable
• Depress the brake pedal at least twenty times or until the pedal goes hard. Optionally check that the low hydraulic pressure warning switch (WR - below accumulator, MB/RR - diagonally mounted on brake servo) has closed
• Remove cap on brake fluid reservoir. Optionally check that the brake fluid level warning switch in the cap closes - this is now a common failure point and a safety issue. The brake fluid reservoir should NOT be topped up during normal service - the falling level is a good indicator of pad wear and the level alarm will trigger before the pads are completely worn.
• If changing fluid completely (every two years) drain brake fluid from reservoir using a syphon (brake fluid is poisonous) or a MityVac pump, open caliper bleed nipples in turn and pump fluid out with brake pedal.
• Attach a pressure system such as a VAG 1238/1 or Gunson Eezibleed (2 to 2.5 bar) to the master cylinder reservoir - Note 1
• Use either the pedal depressor shipped with the 1238/1 or a broom braced against the driver's headrest to hold the brake pedal depressed.
• Bleed the system in the following order:
  • Master cylinder.
  • Proportioning valve (check for leaks on the front of the valve).
  • Rear right caliper.
  • Rear left caliper.
  • Front right caliper.
  • Front left caliper.
  • At least 500ml of brake fluid must pass down each line.

9mm and 11mm spanners or specialist brake spanners (e.g., Williams Superslim) are required. Recently shipped (2007) calipers sometimes have 7mm bleed nipples. Access to the 9mm bleed nipple on the proportioning valve is restricted. For the Coupe quattro, an 11mm Crowfoot socket is optimal.

Note 1: The technique described above uses a pressure bleeder as documented in Audi's repair guidelines. This not the only and perhaps not even the best way to do it - Audi's aim was to make it a one-man operation to save service costs. However, it is perfectly possible to bleed quattro brakes the conventional way by depressing the pedal with the relevant bleed nipple open, closing the bleed nipple, releasing the brakes and repeating the procedure. This obviously requires an assistant operating either the brake pedal or the bleed nipple.

It's possible to make up a device to save the need for repeated opening and closing of the bleed nipple:

• Obtain a 30cm length of thick-walled hose to fit tightly over the bleed nipple
• With a very sharp instrument such as a razor blade or scalpel, cut a slit along the length of the tube about 1cm long ending about 1cm from one end of the tube
• Block that end of the tube with a screw or bolt
• Place the blocked end in a suitable receptacle such as a jam jar
• Attach the other end of the tube to the bleed nipple

Open the bleed nipple and pump the brakes slowly, keeping an eye on the brake fluid reservoir level. The slit acts as a one-way valve - when brake fluid is forced down the tube the slit opens to allow it into the jar; when the pressure is released the slit closes and no fluid or air can get back into the system. When sufficient fluid has been forced through (Audi recommend 500ml per caliper) close the bleed nipple.

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