Use only brake fluid or brake cylinder paste as an aid to installation. Substances containing mineral oil will destroy the rubber elements In the brake system and cause brake failure. In the case of stepped brake master cylinders, there is a washer under the front sealing plug (intermediate piston brake circuit).

4. Bleed the brake/clutch with the filling and bleeding device. Check for leaks and for correct operation.

5.Remove cover and unscrew mounting nuts of brake booster.


1.Remove master cylinder (see page 47 - 13)

2.Remove brake pressure regulator from wheel arch (only cars earlier than model year 84).

4.So that brake pressure rod (piston rod) protrudes as little as possible toward the footwell from the booster, adjust adapter, depress brake pedal again and correct position of hose clamp. Remove adapter.

3.Depress brake pedal fully and using a hose clamp, fix pressure rod for master cylinder in this position. Place a strip of 2 mm sheet beneath clamp.

Printed in Germany - XIII, 1986 Removing and Installing Brake 47 - 15


6.If necessary, unclip brake line from right-hand front wheel from its holder on bulkhead and carefully push toward engine. Position hose for clutch release cylinder and electric cables so that they do not obstruct during removal.

3.Install brake pressure regulator and master cylinder. Always renew seal ring between brake booster and master cylinder. Mount brake pressure rod with adapter on brake pedal. Adjust pressure rod play and check setting of mechanical stoplamp switch, if fitted (page 46 - 10a).

4.Bleed brakes/clutch with charging and bleeding equipment. Check for leaks and correct operation.

7.Remove brake booster. It may be necessary to make space by removing the lower section of the air filter to allow passage of a 10" brake booster.

1.Renew seal ring between bulkhead and brake booster if damaged. The 10" boosters are fitted with 2 mm seals, or 1 mm seals as of model year 85. Use a 4 mm seal or two 2 mm seals for 9" boosters.

2.Install brake booster and tighten securing nuts. Check that the cover over the control housing is correctly seated.

47 - 16 Removing and Installing Brake XIII, 1986 - Printed in Germany Booster


Regulator on Wheel House

Removing Installing

1. Remove intake hoses and air cleaner upper section.

2. Drain both chambers of brake fluid tank by pumping out brake fluid through bleeder valves of front and rear wheel brake calipers {diagonal brake circuit division).

1. Check brake pressure regulator/car application on page 47-18.

2. Install brake pressure regulators. The mounting plate has a retaining tab for each brake pressure regulator to guarantee correct installed position and prevention of turning when tightening bolts

3. Detach brake lines. Remove brake pressure 3. Connect brake lines. Bleed brakes, regulator after unscrewing mounting bolt(s).

I Regulator for right rear wheel

II Regulator for left rear wheel

If applicable, cancel the brake circuit failure indicator lamp by disconnecting the battery (ground wire on body).

Screw-in Regulator (since 1984 models)

Cars with ABS: connect h screwed in hydraulic unit.

Ca rs without ABS: screwed in master cylinder (intermediate piston circuit).

1 — Inlet of intermediate piston brake circuit

2 - Outlet to right rear wheel Removing

3 — Inlet of push rod brake circuit

4 - Outlet to left rear wheel 1- Remove left intake hose.

2. Unscrew brake line on brake pressure regulator, counterholding on hexagon of regulator.



1. Screw in brake pressure regulator. Mount brake line on brake pressure regulator while counterholding on the hexagon.

Place cloths underneath to avoid damage from escaping brake fluid.

2. Bleed brakes (begin at bleeder valve of master cyl.}.

3. Disconnect brake pressure regulator on brake master cylinder or hydraulic unit, never turning on the hexagon. Plug open connection with a plug.


Braking-Force Regulators Until End of Model Year 83

(Diagonal braking circuits/2 regulators per vehicle)

9" Brake Boosters previously replace by installed

10" Brake Boosters previously installed replace by

Switchover pressure in bar

Part No.

Identification (color + switchover pressure)

Switchover pressure & reduction factor (stamped on cap)



Painted black



Painted black green dot in center of cap


When replacing only use 928.355.083.02 as a pair. Under no circumstances may 33 and 55 bar regulators be used together in a car.

Switchover pressure and reduction factors stamped on cap.

55 or 33 = switchover pressure (bar)

5 = reduction factor 0.46

* For a brief period, brake pressure regulators painted black and marked only with a blue dot were installed. Note switchover pressure on cap.

(928.355.083.01) discontinued

Painted blue*


Painted black, green dot in center of cap



When replacing only use 928.355.083.02 as a pair. Under no circumstances may 928. 355.083.01 and 928.355.083.02 regulators be used together in a car. I

Printed in Germany - XIII, 1986 Braking-Force Regulators - 47 - 18a

Installation Overview (until End of Model Year 83)

Braking-Force Regulators, 84 Models Onward

(stage-type tandem master cylinder, separate brake circuits for each axle, 1 screw-in regulator per car)

Installed in

Part No.

Switchover pressure

84 models

85 models

86 models onward (with four-piston fixed-

928.355.305.01 951.355.305.00

33 bar 33 bar 18 bar


Cars with ABS: Screwed into port h of hydraulic unit.

Cars without ABS: Screwed into master cylinder (inter-piston circuit).


Switchover pressure and reduction factor stamped on regulator.

33 or 18 = switchover pressure (bar) 5 = reduction factor 0.46

It is essential to ensure that the appropriate parts are installed. Under no circumstances may braking-force regulators with different reduction factors (e.g. 3 = 0.3) be installed.

47 - 18b Braking-Force Regulator - Installation Printed in Germany Overview (84 Models Onward)



A braking-force regulator is integrated in the brake lines to the left-hand and right-hand rear wheels (diagonal braking circuits). This version was retained until the end of model year 1983. From model year 1984 onward, only one brakingforce regulator is installed for both rear wheels (separate braking circuits for each axle). This regulator is smaller and lighter. It is screwed directly into the modified master cylinder or into the hydraulic unit of vehicles with ABS (screw-in regulator).

The braking-force regulators have a permanently set switchover pressure of 18, 33 or 55 bar (see overview, page 47 - 18a/18b)- Until pressure rises to the switchover point, the same overpressure is applied to the input and output sides of the braking-force regulator. As the pressure on the input side rises further, the output overpressure (rear-axle overpressure) is reduced.

Preparati on

1.Connect brake calipers concerned and pressure tester by screwing test adapters into holes for bleeder valves in brake calipers.

Diagonal ly-spl it Brake Circuits

Test connections for braking-force regulator of left-hand rear wheel: on the left behind and on the right in front of the right-hand rear wheel :

rear right and front left

Separate Braking Circuits for Each Axle (84 models onward)

Test connections for the screw-in regulator:

front axle left or right and rear axle left or right.

To check the braking-force regulators of diagonally-split braking circuits, the pressure which is not reduced (front axle) and the reduced pressure (rear axle) must be read simultaneously on manometers installed independently in the separate circuits.

To check the braking-force regulator (screw-in regulator) of brakes with separate circuits for each axle, the pressure which is not reduced (front axle) and the reduced pressure (rear axle) must be read simultaneously on independent manometers in the separate circui ts.

Test I

Build up input overpressure (shown on manometer) of front brake caliper (unregulated wheel) in accordance with fable. Read output overpressure on manometer of rear brake caliper (regulated wheel) (specified reading, see table below).


If the input overpressure (unregulated wheel) is inadvertantly exceeded, do not reduce pressure to adjust. Pressure must be built up again from 0 bar (also applies for test II).

2.Connect line from tester (miniature coupling) to test adapter.

3.Bleed test lines and manometers of pressure tester.

4.Check switchover pressure of braking-force regulator installed in car (18, 33 or 55 bar). Before testing, check against table (page 47 - 18a/18b) to ensure that correct regulator is installed.

Increase input overpressure (shown on manometer) of front brake caliper (unregulated wheel) to 100 bar. Read output overpressure on manometer of rear brake caliper (regulated wheel).

Switchover pressure of brake regulator

Input overpressure (front axle)

Output overpressure (rear axle)

Regulator at wheel arch

55 bar 33 bar

55 bar 33 bar

70 bar 48 bar

100 bar 100 bar

Screw-in regulator 33 bar 48 bar 18 bar 33 bar

33 bar 18 bar

100 bar 100 bar



















Adjustment l.If the measured values do not correspond to those stated in the table or if the pressure in the regulated circuit increases while the force exerted on the brake pedal remains constant, renew the braking-force regulator. When replacing, see note on page 47 - 18a/18b.

2.After testing and removal of the equipment, bleed the brake system.

3.Reset the brake circuit failure warning by disconnecting the battery (ground lead to body).

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