Brake System Diagnosis and Repair Chapter 72 - Part One
Brake System Inspection • Brake pedal reserve distance is measured from the vehicle’s floor to the brake pedal when the brake is applied. • Brake pedal height is the distance from the vehicle’s floor to the brake pedal when the brake is not applied. • Free play is the amount of pedal movement before the beginning of brake application. Without free play brakes may be slightly applied even without depressing the pedal, causing overheating and grabbing.
Brake System – Brake Fluid Inspection • An important part of a brake inspection includes checking the condition and level of the brake fluid. • Transmission fluid can never be used in place of brake fluids. • Brake fluid should be kept at least 1-1/2 inches from the top of the fluid reservoir. • You should always check for leaks if the brake fluid is low. • Air in the brake lines can cause a spongy ineffective brake pedal. The brakes need to be bled to purge the unwanted air.
Vacuum Booster Service Testing the vacuum booster for correct operation includes these 4 steps: • Step 1 Depress the brake pedal 4-5 times to release stored vacuum • Step 2 Press the brake pedal and hold it down • Step 3 Start the engine • Step 4 After engine starts, pedal should drop about one inch The parts that must be removed to replace a vacuum booster are: Master Cylinder Brake pedal push rod pin Vacuum hose Mounting nuts
Master Cylinder Service • Parts of the Master Cylinder:
Brake System Bleeding - Procedure • Raise and secure front and rear end of vehicle. Clean top of brake master cylinder and remove cover. Determine correct type of brake fluid and fill master cylinder with fresh fluid to full mark. Repeat the following procedure until only clean brake fluid comes out with no air bubbles, Check fluid level often and keep full: • 1. Open brake bleeder ½ turn on right rear wheel • 2. Have helper slowly press brake pedal to floor and hold • 3. Gently close right rear brake bleeder • 4. Have helper release brake pedal • 5. Repeat steps 1-4 until only clean brake fluid comes out with no air bubbles. Recheck and fill brake fluid reservoir Repeat the above steps for left rear, right front, and left front wheels in this specific order. Top up brake fluid when done.
Brake System Bleeding - Procedure • When pressure bleeding the brake system, you must fill the pressure bleeder with brake fluid and pressurize the tank to 32 psi. • You should flush the brake fluid every two years regardless of mileage • All of the following are used as brake fluid: • DOT 3 Brake Fluid (Most common) • DOT 4 Brake Fluid (Less common) • DOT 5 Brake Fluid (Rare) • Hydraulic Mineral Oil (Rare)
Brake System – Types of Brake Fluid • One of the important characteristics of brake fluid is its boiling point. Hydraulic systems rely on an incompressible fluid to transmit force. Liquids are generally incompressible while gases are compressible. If the brake fluid boils (becomes a gas), it will lose most of its ability to transmit force. This may partially or completely disable the brakes. To make matters worse, the only time you are likely to boil your brake fluid is during a period of prolonged braking, such a drive down a mountain -- certainly not the best time for brake failure! • The three main types of brake fluid now available are DOT3, DOT4 and DOT5. • DOT3 and DOT4 are glycol-based fluids, and DOT5 is silicon-based. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't. Your brake fluid plays a big role in how your brakes function. From the website: How Stuff Works
Brake System – Additional Information • You can make your own brake lines. • Brake lines are bled to get the air out. • You always use double walled steel tubing to make brake lines. • Brake fluid should be able to absorb water. • You don’t need to pump the brake pedal when pressure bleeding. • Brake fluid such as Hydraulic Mineral Oil can be green in color.
Questions? • See Mr. Bloom or Mr. Kessel • Now you are ready to proceed to Chapter 72 Part Two