Disc Brakes Audi A6 C4

DESCRIPTION & OPERATION

All models have self-adjusting front and rear disc brakes. A pressure regulator is used to prevent premature lock-up of rear brakes. The cable actuated parking brake mechanically activates the rear calipers.

Hydraulic system uses a tandem master cylinder with vacuum assist. Hydraulic system is diagonally split. Left front and right rear wheels form one circuit. Right front and left rear wheels form the other circuit.

BLEEDING PROCEDURES

1. Ensure brake master cylinder reservoir is full. Connect hose to brake caliper bleed screw and insert other end of hose in a clear container 1/4 full of clean brake fluid. Bleed brake calipers in sequence. See «BRAKELINE BLEEDING SEQUENCE»(/audi/a6/c4-1994-1997/remont/mechanical-hydraulic/#disc-brakes) table. BRAKELINE BLEEDING SEQUENCE Application Sequence All Models RR, LR, RF & LF
2. On vehicles with height-sensitive brake pressure regulator, press regulator lever firmly toward rear axle when bleeding. About 1/2 qt. (500 cc) of brake fluid is expelled from each wheel when bleeding entire system. Before driving vehicle, depress brake pedal several times to ensure brakes are working correctly.

BRAKE PRESSURE REGULATOR

Note. Information is not available at time of publication for Quattro models.

Brake hydraulic system must be properly bled. Raise and support vehicle allowing rear springs to extend fully. Compress brake pressure regulator lever against stop. Loosen adjustment nut. Insert 11 mm (.43") drill bit between spring and support boss. (Scheme 1) Tighten adjustment nut and remove drill bit. Tighten lock nut. Ensure rear wheels rotate.

Scheme 1

BRAKELIGHT SWITCH

With brake pedal released, measure distance "A". (Scheme 2) If distance is not.004-.020" (0.1-0.5 mm), loosen lock nut and adjust as necessary.

Scheme 2

POWER BOOSTER PUSH ROD

Measure push rod length whenever power booster or master cylinder is replaced. Push rod length is measured from master cylinder mounting flange to center of clevis pin. Measure distance "A". (Scheme 3) Adjust push rod length if not as specified. See POWER BOOSTER PUSH ROD LENGTH table.

POWER BOOSTER PUSH ROD LENGTH

Scheme 3

PARKING BRAKE

Note. Adjust parking brake only if parking brake cable, rear brake pads or rear caliper have been replaced.

1. Raise and support vehicle. Release parking brake lever. Locate parking brake adjustment nut near exhaust shield. (Scheme 4) Loosen adjustment nut until parking brake levers contact boss on caliper. Turn adjustment nut back 2 turns.
2. Press parking brake cable lever in contact with caliper boss. If lever of opposite caliper pulls away from stop, cable is too tight. Loosen adjusting nut until both brake levers are just resting on stops. Release parking brake. Ensure both wheels rotate freely.

Scheme 4

Front

All wheel bearings are press fit into wheel bearing housing and require no lubrication or adjustment. Replace wheel bearing if failure occurs. Wheel bearing must be replaced if removed from housing.

Rear (Quattro)

All wheel bearings are press fit into wheel bearing housing and require no lubrication or adjustment. Replace wheel bearing if failure occurs.

Rear (Except Quattro)

Rotate wheel and tighten adjusting nut to seat bearing. When all play is eliminated, back off adjusting nut until thrust washer can be rotated with slight effort using a screwdriver.

Note. Information is not available at time of publication for Quattro models.

Pressure Test

1. Ensure hydraulic system is free of air. See «BLEEDING BRAKE SYSTEM»(/audi/a6/c4-1994-1997/remont/mechanical-hydraulic/#disc-brakes) . Vehicle should be on level surface and emergency brake set.
2. Locate brake pressure regulator on left rear axle. Firmly apply and release brake pedal. Ensure lever on brake pressure regulator moves. If lever does not move, replace brake pressure regulator.

Leakage Test

1. If leakage is suspected in brake pressure regulator, connect Pressure Gauges (US 1016) to left front caliper and right rear caliper. Bleed pressure gauges and hoses through bleed screws on gauges. Pump brake pedal until pressure at front caliper reaches 1450 psi (101.9 kg/cm 2 ).
2. Hold pressure for 5 seconds. If rear pressure reading changes by more than 145 psi (10.2 kg/cm 2 ) during 5 seconds when pedal is held, brake pressure regulator is defective. Replace regulator and bleed brake system.

Removal

Raise and support vehicle. Remove front wheels. Remove caliper guide pins or self-locking bolts. Disconnect brake hose from caliper. Plug openings in hose end and caliper. Remove caliper from mounting frame.

Installation

To install, reverse removal procedure. Use NEW self-locking bolts (if equipped) and heat shields. Bleed complete hydraulic system. See BLEEDING BRAKE SYSTEM .

Inspection

Measure brake pad thickness, including friction material and backing plate. Minimum brake pad thickness is .275" (7.0 mm).

Press piston back into caliper. Install brake pads and NEW heat shield. Position caliper in carrier. Tighten housing guide bolts to 18 ft. lbs. (25 N.m). Pump brake pedal several times to adjust piston and brake pads to rotor.

1. Raise and support vehicle. Remove front wheels. Remove brake pad wear indicator plug from retainer clip. Compress plug at knurled surface and disconnect indicator lead.
2. Hold guide pin using open-end wrench and remove lower caliper self-locking mounting bolt. Pivot caliper up on upper mounting bolt. Remove both pads from pad carrier. (Scheme 5)

Scheme 5

Measure brake pad thickness, including friction material and backing plate. Minimum brake pad thickness is .275" (7.00 mm).

1. Press piston back into caliper. Install brake pads with NEW heat shield between inner pad and piston. Ensure heat shield faces piston. Swing caliper down into carrier.
2. Install NEW lower self-locking mounting bolt. Tighten self-locking bolt to 26 ft. lbs. (35 N.m). Pump brake pedal several times to adjust piston and brake pads to rotor.

Removal & Installation

Raise and support vehicle. Remove front wheels. Remove caliper and hang it from frame, leaving brake hose connected. Remove pad carrier from wheel bearing housing. Slide rotor off hub. To install rotor, reverse removal procedure.

Raise vehicle and remove rear wheels. Remove caliper guide pins or self-locking bolts. Disconnect brake hose from caliper. Plug openings in hose end and caliper. Disengage caliper from carrier. (Scheme 6)

To install, reverse removal procedure. Use NEW self-locking bolts (if equipped) and heat shields. Bleed hydraulic system.

Scheme 6

Raise and support vehicle. Remove rear wheels. Hold guide pin using open-end wrench while removing self-locking mounting bolt. (Scheme 6) Remove caliper and wire aside. DO NOT disconnect or damage hydraulic line. Remove brake pads from carrier.

Measure brake pad thickness, including friction material and backing plate. Minimum brake pad thickness is .275" (7.00 mm).

Turn piston clockwise while pushing it firmly into caliper bore. Install brake pads in carrier. Install caliper using NEW self-locking mounting bolts. Tighten mounting bolts to 26 ft. lbs. (35 N.m). Always check basic setting of rear brakes and parking brake.

Raise vehicle and remove rears wheels. Remove caliper and wire aside. Remove pad carrier from wheel bearing housing. Slide rotor off hub. To install rotor, reverse removal procedure.

MASTER CYLINDER

Note. DO NOT allow brake fluid to contact painted surfaces.

Disconnect steel hydraulic lines at master cylinder, and plug ports to prevent fluid spillage. Remove mounting bolts. Separate master cylinder from power assist brake servo. DO NOT pull out servo rod.

To install, reverse removal procedure. Replace "O" ring between master cylinder and power assist brake servo. Lubricate rubber grommets between reservoir and master cylinder. Bleed hydraulic system. See BLEEDING BRAKE SYSTEM .

Release system pressure. Remove master cylinder. Pull pin holding operating rod clevis to brake pedal. Remove self-locking nuts holding servo to firewall. Disconnect vacuum supply line. Remove power booster.

To install, reverse removal procedure. Adjust push rod length. See POWER BOOSTER PUSH ROD under ADJUSTMENTS. Use NEW self-locking nuts and NEW "O" ring seal between servo and firewall bracket. Roll seal against firewall after tightening nuts holding servo to bracket.

To install, reverse removal procedure. Adjust push rod length. See POWER BOOSTER PUSH ROD under ADJUSTMENTS. Lightly grease clevis pin. Use NEW "O" ring between master cylinder and servo. Use NEW self-locking nuts. Bleed brakes and check system operation.

Disassembly

Remove caliper from vehicle. Remove pads, and clean exterior surfaces. Use air pressure to force piston from cylinder. Use a block of wood between piston and housing to prevent damage to piston. Remove dust seal. Remove piston seal without damaging bore or groove. (Scheme 7)

Scheme 7

Cleaning & Inspection

Use alcohol or brake cleaning fluid to clean hydraulic chambers. Check cylinder bore and piston for corrosion and damage. Replace defective parts as necessary.

Reassembly

1. Use all parts supplied in repair kit. Coat piston, cylinder bore and NEW seal with clean brake fluid. Fit piston seal into cylinder. Slide dust seal onto piston. Slowly insert piston into bore.
2. Seat inner lip of dust seal in groove on cylinder housing. Open bleed screw and push piston into bore as far as possible. Fit outer lip of dust seal into piston groove.

Remove pads, and clean outside surfaces. Using Allen wrench, unscrew piston from cylinder. Carefully remove piston seal without damaging bore or groove. (Scheme 8)

Scheme 8

Use alcohol or brake parts cleaning fluid for cleaning purposes. Check cylinder bore and piston for wear and corrosion.

Use all parts supplied in repair kit. Coat piston, cylinder bore and NEW seal with brake fluid. Fit piston seal into cylinder. Slide dust seal into groove on piston. Press down on piston while screwing it into cylinder bore. Ensure outer lip of dust seal slips into piston groove.

Manufacturer recommends replacement of master cylinder if defective.

VACUUM POWER BOOSTERS

Manufacturer recommends replacement of vacuum power booster if defective.

TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS

DISC BRAKE SPECIFICATIONS

DISC BRAKE SPECIFICATIONS

See also:
• BRAKELINE BLEEDING SEQUENCE
• POWER BOOSTER PUSH ROD