A hydraulic system is used to actuate the brakes. The system transports the power required to force the frictional surfaces of the braking system together, from the pedal to the individual braking units at each wheel.
A hydraulic system is used for three reasons. First, fluid under pressure can be carried to all parts of the automobile by small tubes and hoses, some of which are flexible, without taking up a significant amount of room or posing routing problems. Second, liquid is non-compressible; a hydraulic system can transport force without modifying or reducing that force. Third, 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 either the wheel cylinders or calipers.
The master cylinder consists of a fluid reservoir, a double cylinder and a piston assembly. Double type master cylinders are designed to separate the braking systems diagonally in case of a leak.
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 cylinder by flexible tubes in order to allow for suspension and steering movements.
In drum brake systems, the slave cylinders are called wheel cylinders. Each wheel cylinder contains two pistons, one at either end, which push outward in opposite directions.
In disc brake systems, the slave cylinders are part of the calipers. One large cylinder is used to force the brake pads against the disc. All slave cylinder pistons employ some type of seal, usually made of squared off rubber, to prevent fluid leakage from 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 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 pistons 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 pistons 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.
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 drilled compensating ports. 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 master cylinder pistons withdraw. Then, as the return springs force the shoes into the released position, the excess fluid returns to the master cylinder 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 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.
All dual circuit systems use a warning 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 master cylinder. A hydraulic piston receives pressure from both circuits, each circuit's pressure being applied to one end of the piston. When the 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 distributor switch and activating the brake warning light.
In 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 drums, ensuring that the front brakes will never be used alone. The proportioning valve throttles the pressure to the rear brakes so as to avoid rear wheel lock-up during very hard braking.
These valves may be tested by removing the lines to the front and rear brake systems and installing special brake pressure testing gauge. Front and rear system pressures are then compared as the pedal is gradually depressed. Specifications vary with the manufacturer and design of the brake systems.
Brake warning lights may be tested by depressing the brake pedal and holding it while opening one of the wheel cylinder bleeder screws. If this does not cause the light to go on, substitute a new lamp, make continuity checks, and finally, replace the switch as necessary.
The hydraulic system may be checked for leaks by applying pressure to the pedal gradually and steadily. If the pedal sinks very slowly to the floor, the system has a leak. This is not to be confused with a springy or spongy feel due to the compression of air within the lines. If the system leaks, there will be a gradual change in the position of the pedal with a constant pressure.
Check for leaks along all lines and at each wheel. If no external leaks are apparent, the problem may be within the master cylinder.
Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems utilize a cast iron disc with brake pads positioned on either side of it. Braking effect is achieved in a manner similar to the way you would squeeze a spinning phonograph record between your fingers. The disc (rotor) is a one-piece casting with cooling fins between the two braking surfaces. This enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade. Dirt and water do not affect braking action since contaminants are thrown off by the centrifugal action of the rotor or scraped off by the pads. Also, the equal clamp action of the two brake pads tends to ensure uniform, straight-line stops. All disc brakes are self-adjusting.
Drum brakes employ two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular cast iron drum which rotates with the wheel assembly. The shoes are held in place by springs; this allows them to slide toward the drums (when they are applied) while keeping the linings and drums in alignment. The shoes are actuated by a wheel cylinder which is mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's two actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced outward against the inner side of the drum. This action forces the bottoms of the two shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). When pressure within the wheel cylinder is relaxed, return springs pull the shoes back away from the drum.
The drum brakes are designed to self-adjust during application when the car is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever, thereby causing rotation of the adjusting screw by means of an actuating lever.