Braking system

Ford Sierra / Ford Sierra Service Manual / Braking system

General information and precautions

General information
The braking system is of the dual circuit hydraulic type. The front and rear circuits are operated independently from a tandem master cylinder, so that in the event of a hydraulic failure in one circuit, full braking force will still be available to two wheels through the remaining circuit.

A deceleration sensitive valve on Saloon, Hatchback and Estate models not fitted with an Anti-lock Braking System (ABS), and a load apportioning valve on P100 models, is incorporated in the rear brake hydraulic circuit. The valve regulates the pressure applied to the rear brakes and reduces the possibility of the rear wheels locking under heavy braking.

All models are fitted with front disc brakes, with solid or ventilated discs depending on model. The calipers are of single piston sliding type, which ensures that equal pressure is applied to each disc pad.

Non-ABS models are fitted with rear disc brakes or rear drum brakes, incorporating leading and trailing shoes operated by double-acting wheel cylinders. A self-adjuster mechanism is fitted which consists of a toothed quadrant which is kept in contact with a toothed pin attached to the shoe strut by means of a spring. The quadrant incorporates an arm which locates in a slot in the leading shoe. As the shoe linings wear the quadrant is pulled from the pin when the footbrake is operated, and automatically repositioned to effectively lengthen the shoe strut.

ABS is available as an option for all models except the P100. The system comprises an electronic control unit, roadwheel sensors, hydraulic actuator with electrically-driven hydraulic pump, and the necessary valves and switches. Disc brakes are fitted to all four wheels. The front disc brakes are similar to those fitted to non-ABS models, but the rear brakes incorporate a self-adjusting mechanism, and a mechanical handbrake mechanism. The purpose of the system is to prevent wheel(s) locking during heavy brake applications. This is achieved by automatic release of the brake on the locked wheel, followed by reapplication of the brake. This procedure is carried out four times per second by the control valves in the valve block. The valves are controlled by the electronic control unit which itself receives signals from the wheel sensors, which monitor the locked or unlocked state of the wheels. A pressure regulating valve is incorporated in the rear hydraulic circuit to maintain the desired pressure ratio between the front and rear circuits.

Precautions
Note: Hydraulic fluid is poisonous; wash off immediately and thoroughly in the case of skin contact and seek immediate medical advice if any fluid is swallowed or gets into the eyes.

Certain types of hydraulic fluid are inflammable and may ignite when allowed into contact with hot components; when servicing any hydraulic system it is safest to assume that the fluid is inflammable and to take precautions against the risk of fire as though it is petrol that is being handled. Hydraulic fluid is also an effective paint stripper and will attack plastics; if any is spilt, it should be washed off immediately using copious quantities of fresh water. Finally, it is hygroscopic (it absorbs moisture from the air) old fluid may be contaminated and unfit for further use. When topping-up or renewing the fluid, always use the recommended type and ensure that it comes from a freshly-opened sealed container
Note: When working on the brake components, take care not to disperse brake dust into the air, or to inhale it, since it may contain asbestos which is injurious to health.

Braking system

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