BMW Coupes and Sedans 1970-1988 Repair Guide

Basic Operating Principles



See Figures 1 and 2

Hydraulic brake systems are used to actuate the brakes for most modern automobiles. The system transports the power required to force the frictional surfaces together from the pedal to the individual brake units at each wheel. A hydraulic brake system allows fluid under pressure to be carried to different parts of the vehicle by small hoses without taking up a significant amount of room.

The master cylinder is the controlling mechanism in the hydraulic braking system. It consists of a fluid reservoir and either a single or double cylinder piston assembly. Double type master cylinders are designed to separate the front and rear braking systems hydraulically in the event of a fluid leak.

Steel lines carry the brake fluid from the master cylinder along the vehicle frame to flexible hoses attached to either calipers or wheel cylinders depending on the type of brake and location.

If equipped with wheel cylinders, each wheel cylinder contains two pistons, one at either end, which push outward in opposite directions. If equipped with calipers, pistons are encased in the caliper and force the brake pads against the disc. All calipers employ some type of seal around the piston, usually made of rubber, to minimize fluid leakage. A rubber dust boot seals the outer end of the piston against dust and dirt.

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Fig. Fig. 1: Diagram of a standard brake system

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Fig. Fig. 2: Diagram of a hydraulically boosted dual diagonal brake system

When at rest, the entire hydraulic system, from the piston(s) in the master cylinder to those in the wheel cylinders or calipers, is full of brake fluid. Upon application of the brake pedal, fluid trapped in front of the master cylinder piston(s) is forced through the lines to the wheel(s) and forces the pistons outward

Upon release of the brake pedal, a spring located inside the master cylinder immediately returns the master cylinder pistons to its normal position.

Dual circuit master cylinders employ two pistons, located one behind the other, in the same cylinder. The primary piston is actuated directly by mechanical linkage from the brake pedal. The secondary piston is actuated by fluid trapped between the two pistons. If a leak develops in front of the secondary piston, it moves forward until it bottoms against the front of the master cylinder, and the fluid trapped between the pistons will operate the rear brakes. If the rear brakes develop a leak, the primary piston will move forward until direct contact with the secondary piston takes place, and it will force the secondary piston to actuate the front brakes. In either case, the brake pedal moves farther when the brakes are applied, and less braking power is available.

Most dual-circuit systems use a switch to warn the driver when only half of the brake system is operational. This switch is located in a valve body which is mounted on the firewall or the frame below the master cylinder. When the braking pressures are in balance, the piston remains stationary. When one circuit has a leak, however, the greater pressure in that circuit during application of the brakes will push the piston to one side, closing the switch and activating the brake warning light.

In some disc brake systems a metering valve and, in some cases, a proportioning valve is incorporated into the braking system. The metering valve keeps pressure from traveling to the disc brakes on the front wheels until the brake shoes on the rear wheels have contacted the drums, ensuring that the front brakes will never be used alone. The proportioning valve controls the pressure to the rear brakes to avoid rear wheel lock-up during very hard braking.


See Figure 3

Certain BMW models are equipped with an Anti-lock Brake System (ABS). This system functions in the same manner as the standard hydraulic system, but is more advanced in that it is designed to control the braking of each individual wheel under different braking situations. Basically, the system monitors the speed of each wheel as the brakes are applied. It does this with a sensor at each wheel. When the system determines that one or more wheels are losing traction under braking, it modulates the braking force to the wheel(s) which prevents the wheel(s) from locking up and the vehicle possibly going out of control.

The ABS system requires no additional service with the exception of disconnecting the negative battery cable prior to performing any brake service. This will to protect the control unit.

If ABS system failure is suspected or the ABS warning indicator stays on, the vehicle should be taken to an authorized service station. ABS testing requires very expensive electronic testers and special troubleshooting procedures.

The ABS system consists of a control unit, hydraulic unit, speed sensors and related wiring. The following is a description of each and its location:

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Fig. Fig. 3: Diagram of an anti-lock brake system

Speed Sensors

Located behind each wheel installed in the wheel hub is a gear which runs past a permanently magnetized edge of the sensor. This rotary motion of the wheel is recorded and the electrical signal is sent to the control unit.

Electronic Control Unit

The control unit is located above the pedal assembly on the driver's side of the vehicle. The control unit reads and interprets various wheel speed data from the speed sensors. This data is then used to determine wheel speed, loss of traction and excess grab. It takes all the data into account and adjusts the electronically modulated valves of the hydraulic unit accordingly. This enable the driver to maintain complete control of the vehicle under all operating conditions. All of the operation of the control unit takes place in milli-seconds. ABS operation can be felt, as a brake pedal pulsation by the driver, under extreme braking conditions. There is also a warning light on the dash to indicate if the system is operating, if the light stays on, there is a problem with the system.

In the event of system failure normal braking will be maintained.

Hydraulic Unit

This is a hydraulic distributor and actuator in series with the master cylinder and brake lines. It consists of electronically controlled valves that supply brake fluid to each individual wheel, as dictated by the ABS control unit. The hydraulic unit also has an electronic return pump which transfers the brake fluid to the master cylinder while in operation. If the pump or the hydraulic unit fails, normal braking will be maintained. The hydraulic unit can be found in the engine compartment in front of the master cylinder.


Vacuum assisted or power brakes as they are most commonly called operate just as standard brake systems except in the actuation of the master cylinder pistons. A vacuum diaphragm is located on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel needed in moving the brake pedal.

The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brake assist vacuum will not be lost.

Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm, and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod, and direct mechanical application will occur as the pedal is depressed.


The hydraulic system may be checked for leaks by applying pressure to the pedal gradually and steadily. If the pedal sinks slowly to the floor, the system may have a leak. This is not to be confused with a springy or spongy feel due to the compression of air within the lines. In this case the pedal may feel soft, but will not compress to the floor. If the system leaks, there will be a gradual change in the position of the pedal.

Check for leaks along the bake lines, hoses and at each wheel. If no external leaks are apparent, the problem is most likely inside the master cylinder.

Warning lights may be tested by depressing the brake pedal and holding it while opening one of the wheel bleeder screws. If this does not cause the light to go on, substitute a new bulb, make continuity checks, and, finally, replace the switch as necessary.

If your vehicle is equipped with power brakes, and you suspect a vacuum leak, the system may be tested as described below:

  1. Operate the engine at idle without touching the brake pedal for at least one minute.
  3. Turn off the engine, and wait one minute.
  5. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum was present. If there is no vacuum, air is leaking into the system somewhere.

Test for system operation as follows:

  1. Pump the brake pedal (with engine off) until the supply vacuum is entirely gone.
  3. Put a light, steady pressure on the pedal.
  5. Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall toward the floor if constant pressure is maintained on the pedal.