Basic to the operation of any automotive brake system are two well-known principles of physics. They are:
- A liquid in a closed container transmits applied pressure equally in all directions
- Energy cannot be created or destroyed. It can, however, be converted into other forms of energy.
If the applications of these two principles to automotive brake systems are not immediately obvious, do not be concerned. The rest of this section deals with these principles.
To see how the first principle, hydraulics, relates to brakes, consider how the hydraulic brake system works. The driver presses the brake pedal. This pressure is applied to a non-compressible fluid in the system, the "brake fluid", and the fluid transmits the pressure to the wheel circuits.
The fact that the master cylinder applies pressure equally to each wheel channel, is what allows properly adjusted brakes to stop the car evenly.
Another important term to remember is "non-compressible." This means that the fluid pressure from the brake pedal is transmitted through the system as a solid form. Air can be compressed, but a liquid is virtually non-compressible in an automotive application. Air in the system results in a soft pedal and possibly a brake failure.
Another important point to note about brake fluid is that although all brake fluids are non-compressible, they are not all alike. If you look on the label, you will notice that each container of brake fluid has a DOT (Department of Transportation) designation-DOT 3, 4, or 5. Each fluid has its own characteristics.
The difference between DOT 3 and DOT 4 is their boiling point. Both of these are polyglycol based; however, DOT 3, the type specified in most American and Japanese vehicles, has a minimum dry boiling point of 401 degrees Fahrenheit. DOT 4, the type specified for most European cars, has a dry boiling point of 446 degrees. (Dry boiling point means free of water. Water lowers the boiling point of the brake fluid and may affect performance.)
Because glycol-based brake fluids do absorb moisture (hygroscopic), corroding brake parts over time, and damaging painted surfaces, many car enthusiasts have converted their vehicles to DOT 5 silicone brake fluid. It has a boiling point of more than 500 degrees Fahrenheit, does not damage the vehicle's paint and, because it does not absorb water, it will not corrode the brake system components. This means that-all other things being equal, the use of DOT 5 brake fluid will mean a longer life for the cylinders and the hydraulic brake system. That does not mean, however, that DOT 5 brake fluid should be put into every car. DOT 5 (silicone) fluid should never be used on a vehicle equipped with an ABS brake system.
Few manufacturers equip their vehicles with silicone brake fluid and, since silicone brake fluid and glycol-based DOT 3, 4-brake fluid do not mix, the only way to convert to silicone is to completely purge the polyglycol brake fluid from the system by doing a complete brake system overhaul.
The final point to remember about brake fluid is that it does not last forever. Over time, the brake fluid accumulates sediment and moisture. This affects the brake fluid's performance and harms the other components of the hydraulic system.
Car care experts recommend that all brake systems be flushed every two years. This involves purging all of the old brake fluid out of the system and replacing it with new fluid. Although flushing the system is not a complicated operation, you should keep in mind that petroleum products should be kept out of the brake system. If petroleum-based products are introduced into the hydraulic brake system, the rubber seals will swell, creating a problem that can only be solved by a complete overhaul including replacement of all rubber components.
Bleeding the System
After a brake job, air and old fluid must be removed. This is called "bleeding the system". It can be done manually by two people or by one person with a pressure or vacuum bleeder. The air is bled from the system through bleeder screws, located on the uppermost part of the master cylinder (if present), calipers, and wheel cylinders. If a bleeder screw is broken off, it must be repaired or air will remain in the system. Air retained in the system can result in a "soft or spongy pedal" or in a complete loss of pedal.
Since brake fluid absorbs moisture from the atmosphere, it is important to keep the cap on the brake fluid and the cover on the master cylinder. Once moisture enters the hydraulic system, either during repair or because of condensation, it can eventually rust and pit the bore and finish on the cylinder, resulting in frozen or leaking wheel cylinders and calipers.
Every DOT 3 or DOT 4 brake system should be flushed periodically, at least every two years, for best hydraulic system operation.
There are basically three types of hydraulic brake systems in automobiles. Prior to 1967, a single piston master cylinder was used to provide hydraulic brake system pressure to all four wheels simultaneously. This type of system was effective but offered no provision for braking in the event of a failure in any part of the system.
A dual system, or front/rear split, utilizes a dual piston master cylinder that separates, or makes independent of each other, the front and rear hydraulic portion of the system.
A Dual Diagonal System, like the dual system, uses a dual-piston master cylinder and two independent braking systems. The dual diagonal system, however, links the right front and left rear wheels on one part of the system and the left front and right rear are on the other.
In many respects, a brake system is like an energy conversion machine. It takes one type of energy, motion, and converts it into another, heat. That heat is dissipated into the atmosphere. This heat is generated by friction.
Friction can be defined as the resistance to motion between two surfaces touching each other. In a brake system, the two surfaces in drum brakes are brake shoes and linings, and brake drums. In disc brakes, the two surfaces are the brake pads and rotors. It is this resistance to motion that actually stops the vehicle.
It is important to understand how friction works in a brake system-- what creates it and what does it do. Let's take a look at different parts of this stopping formula:
Pressure + Friction Material + Contact Area = Heat
Pressure: The brake system is designed to press the friction material against the braking surface (rotor or drum) and stop the vehicle. The amount of hydraulic pressure in the system is determined by the amount of force used to step on the brake pedal, the bore size of the master cylinder, and the size of the brake line.
In today's brake systems, pressure is converted into two types of mechanical actions: self-energizing and non-energizing.
A brake is called self-energizing if it uses the rotational force of the wheel to help stop the automobile. On this type of brake, the primary shoe contacts the drum, and the force travels through the adjuster link on the bottom to the secondary shoe. The secondary shoe wedges against the drum, stopped by an anchor pin and hydraulic pressure. On a self-energizing brake, the secondary shoe does approximately 70% of the braking. It has a longer lining than the primary shoe.
This type of brake is found on most drum-brake systems.
The non-energizing brake does not use the rotational force of the wheels to help stop the car. With disc brakes, one or more pistons in the caliper press the pads against the rotor, braking the car. On non-energizing drum brakes, a fixed anchor between the brake shoes prevents the rotational force from the leading shoe from transmitting to the trailing shoe. Seventy-percent of the braking action on this type of brake comes from the leading shoe.