Brake Operating System; Basic Operating Principles - Ford Ranger Maintaince And Repair Manual

BRAKE OPERATING SYSTEM

Basic Operating Principles

Hydraulic systems are used to actuate the brakes of all modern automobiles. The system transports the power
required to force the frictional surfaces of the braking system together from the pedal to the individual brake
units at each wheel. A hydraulic system is used for two reasons.
First, fluid under pressure can be carried to all parts of an automobile by small pipes and flexible hoses
without taking up a significant amount of room or posing routing problems.
Second, a great mechanical advantage can be given to the brake pedal end of the system, and the foot pressure
required to actuate the brakes can be reduced by making the surface area of the master cylinder pistons
smaller than that of any of the pistons in the wheel cylinders or calipers.
The master cylinder consists of a fluid reservoir along with a double cylinder and piston assembly. Double
type master cylinders are designed to separate the front and rear braking systems hydraulically in case of a
leak. The master cylinder coverts mechanical motion from the pedal into hydraulic pressure within the lines.
This pressure is translated back into mechanical motion at the wheels by either the wheel cylinder (drum
brakes) or the caliper (disc brakes).
Steel lines carry the brake fluid to a point on the vehicle's frame near each of the vehicle's wheels. The fluid is
then carried to the calipers and wheel cylinders by flexible tubes in order to allow for suspension and steering
movements.
In drum brake systems, each wheel cylinder contains two pistons, one at either end, which push outward in
opposite directions and force the brake shoe into contact with the drum.
In disc brake systems, the cylinders are part of the calipers. At least one cylinder in each caliper is used to
force the brake pads against the disc.
All pistons employ some type of seal, usually made of rubber, to minimize fluid leakage. A rubber dust boot
seals the outer end of the cylinder against dust and dirt. The boot fits around the outer end of the piston on
disc brake calipers, and around the brake actuating rod on wheel cylinders.
The hydraulic system operates as follows: When at rest, the entire 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 cylinders. Here,
it forces the pistons outward, in the case of drum brakes, and inward toward the disc, in the case of disc
brakes. The motion of the pistons is opposed by return springs mounted outside the cylinders in drum brakes,
and by spring seals, in disc brakes.
Upon release of the brake pedal, a spring located inside the master cylinder immediately returns the master
cylinder pistons to the normal position. The pistons contain check valves and the master cylinder has
compensating ports drilled in it. These are uncovered as the pistons reach their normal position. The piston
check valves allow fluid to flow toward the wheel cylinders or calipers as the pistons withdraw. Then, as the
return springs force the brake pads or shoes into the released position, the excess fluid reservoir through the
compensating ports. It is during the time the pedal is in the released position that any fluid that has leaked out
of the system will be replaced through the compensating ports.
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 through the power booster.
BRAKE OPERATING SYSTEM 971