Hydraulic systems are used to actuate the brakes of all modern automotive vehicles. A hydraulic system rather than a mechanical system is used for two reasons. First, fluid under pressure can be carried to all parts of an automobile by small hoses — some of which are flexible — without taking up a significant amount of room or posing routing problems. Second, a great mechanical advantage can be given to the brake pedal, 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 and a single or double cylinder and piston assembly. Double (or dual) 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). Since these components receive the pressure from the master cylinder, they are generically classed as slave cylinders in the system.
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 slave cylinders by flexible tubes in order to allow for suspension and steering movements.
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 slave cylinders are part of the calipers. One, two or four cylinders are used to force the brake pads against the disc, but all cylinders contain one piston only. All slave cylinder pistons employ some type of seal, usually made of rubber, to minimize the leakage of fluid around the piston. A rubber dust boot seals the outer end of the cylinder against dust and dirt. The boot fits around the outer end of either the piston or the brake actuating rod.
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 slave 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 internal springs or spring seals, in disc brakes.
Upon release of the brake pedal, a spring located inside the master cylinder immediately returns the master cylinder piston(s) to the normal position. The pistons contain check valves and the master cylinder has compensating ports drilled within 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. As the return springs force the brake pads or shoes into the released position, the excess fluid in the lines is allowed to re-enter the reservoir 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. The secondary piston is actuated by fluid trapped between the two pistons. If a leak develops in front of the secondary pistons, 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.
All dual-circuit systems incorporate a switch which senses either line pressure or fluid level. This system will warn the driver when only half of the brake system is operational.
In some disc brake systems, this valve body also contains a metering valve and, in some cases, a proportioning valve. 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 drum, ensuring that the front brakes will never be used alone. The proportioning valve controls the pressure to the rear brakes avoiding rear wheel lock-up during very hard braking.