# Description of brake system

### Master cylinder

BMW uses a Tandem master cylinder and dual circuit braking system

The tandem master cylinder has two pistons, actuating two separate fluid circuits when the pedal is depressed.

The dual circuit system supplies brake fluid from one piston to the front left brake and right rear brake, and the second piston to the front right brake and left rear brake. If one system fails, even to the point of total fluid loss from that circuit, the opposing circuit will retain 100 percent efficiency.

Pedal travel will be greatly increased, and total breaking effect will be considerably less than 100 percent, but the chances of total loss of brakes in these systems is highly unlikely.

### Tandem Brake Booster 325iX only

All 325iX vehicles use a tandem vacuum servo brake booster. The tandem unit has been designed for its compact size without loss of power braking assistance.

The purpose of power assist in the brake system is to reduce pedal effort while retaining sensitive pedal control. In the vacuum servo booster, the power assist is provided by the pressure differential between atmospheric pressure on one side of the diaphragm and the vacuum provided from the intake of the engine on the other. The amount of assist depends on the amount of vacuum as well as the surface area of the working piston.

The Tandem Booster uses two pistons, one 8" and the other 9". These pistons operate together as one, combined, surface area. This almost doubles the amount of force that one 9" piston would obtain. This force amounts to approximately 4800N (1079 lb.f) at a vacuum of 0.7 bar ( 10 psi)

As with standard boosters, a non return valve is used to ensure power assist is maintained for several braking operations.

The servo unit is not subject to wear, therefore, maintenance is not required. If replacement of the unit or the master cylinder is required, careful attention must be taken to ensure that the pressure rod of the master cylinder is aligned properly.

The tandem servo brake unit is a self contained unit that basically consists of two vacuum cylinders with a working piston ( diaphragm plate) as well as a control unit. Two diaphragms ( each with one diaphragm plate) divide the cylinders in to chambers where vacuum and atmospheric pressure are applied. Atmospheric pressure is admitted and released from the front (8") chamber through the bellows type seals of the tie rods.

The amount of power boost supplied by the tandem vacuum servo is 5.6:1. This ratio is determined by the difference between the surface area of the sensing piston and the surface area of the reaction piston.

The control unit with valve, actuating rod, reaction disc as well as the reaction piston, transmit and control the servo power.

The servo brake unit develops maximum power when vacuum is applied and full atmospheric pressure is admitted into the boosting chambers. After this point, increased braking pressure is controlled directly by increasing the pedal force.

### Operation

#### Engine running, brakes released

Vacuum is applied to both working chambers. This allows the diaphragm return spring ( via the reaction piston and control housing) to force the sensing piston against the sealing area of the control valve and close off all linked ports to atmospheric pressure.

#### Light braking

When the brake pedal is depressed, the sealing edge of the sensing piston is moved away from the control valve. Atmospheric pressure is now allowed to enter and act upon the diaphragms which move in conjunction with the sensing piston.

#### Heavy braking

With increased brake pedal effort, the opening between the sensing piston and control valve also increases until full atmospheric pressure is obtained

#### Maximum braking

As brake pedal effort is further increased, the limit pin of the sensing piston will contact and move the control housing. This results in the maximum opening between the sensing piston and the control valve

#### Return

Upon completion of the braking operation and release of the brake pedal the stop pin of the sensing piston is pressed against the limit stop on the control housing by the large valve spring. The sensing piston pushes the sealing surface from the sealing edge and a rapid reduction in atmospheric pressure takes place.

E30 M3 Repair Manual V2.7. Copyright ©1999-2001 Koala Motorsport & Brett Anderson