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.
Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems employ a cast iron disc with brake pads positioned on either side of it. An easily seen analogy is the hand brake arrangement on a bicycle. The pads squeeze onto the rim of the bike wheel, slowing its motion. Automobile disc brakes use the identical principal but apply the braking effort to a separate disc instead of the wheel.
The disc or rotor is a one-piece casting mounted just inside the wheel. Some discs are one solid piece while others have cooling fins between the two braking surfaces. These vented rotors enable air to circulate between the braking surfaces cooling them quicker and making them less sensitive to heat buildup and fade. Disc brakes are only slightly affected by dirt and water since contaminants are thrown off by the centrifugal action of the rotor or scraped off by the pads. Also, the equal clamping action of the two brake pads tend to ensure uniform, straight-line stops, although unequal application of the pads between the left and right wheels can cause a vicious pull under braking. All disc brakes are inherently self-adjusting.There are three general types of disc brakes:
- Fixed caliper
- Sliding caliper
- Floating caliper
The fixed caliper design uses two pistons mounted on either side of the rotor (in each side of the caliper). The caliper is mounted rigidly and does not move. This is a very efficient brake system but the size of the caliper and its mounts adds weight and bulk to the car.
The sliding and floating designs are quite similar. In fact, these two types are often lumped together. In both designs, one pad is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly on its mount, bringing the other pad into contact with the rotor. There are various methods of attaching floating calipers. Some pivot at the bottom or top, and some slide on mounting bolts. Many uneven brake wear problems can be caused by dirty or seized slides and pivots.
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. The shoes are held in place by springs; this allows them to slide toward the drum (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 inside 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 the pressure within the wheel cylinder is relaxed, return springs pull the shoes away from the drum.
Most modern drum brakes are designed to self-adjust during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever and thereby causing rotation of the adjusting screw via a star wheel. This on-board adjustment system reduces the need for maintenance adjustments but most drivers don't back up enough to keep the brakes properly set.
Power brakes operate just as non-power 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 he must put into 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 and mechanical problems that apply to conventional brake systems also apply to power brakes, and should be checked for if the tests below do not reveal the problem.Test for a system vacuum leak as described below:
- Operate the engine at idle without touching the brake pedal for at least one minute.
- Turn off the engine, and wait one minute.
- 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.
- Pump the brake pedal (with engine off) until the supply vacuum is entirely gone.
- Put a light, steady pressure on the pedal.
- 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.
Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.