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The principle behind hydraulic brakes is Pascal’s Law, which states that pressure applied to a fluid in one part of a closed system will be transmitted without loss to all other areas of the system. A substantial leak in the hydraulic braking system will prevent enough pressure from building to exert the necessary braking force. Engineers design brakes that have precise (but unequal from front to back) amounts of braking force at each wheel. The three variables related to pressure and force in hydraulic systems are: input force, working pressure, and output force. Main components of the hydraulic braking system are: brake pedal, brake fluid, and master cylinder. The brake pedal multiplies force from the driver’s foot to the master cylinder. Brake fluid has a high boiling point, a low freezing point, and is hygroscopic (absorbs water). Brake fluids are graded by the Department of Transportation on: pH value, viscosity, resistance to oxidation, stability, and boiling point. Master cylinders convert force exerted from the brake pedal into hydraulic pressure to activate wheel brake units. Types of master cylinders are: single piston and tandem (required on modern cars). Single piston master cylinders use a primary cup to seal pressure in the cylinder and a secondary cup to prevent fluid loss. A single piston master cylinder traps brake fluid and forces it into the brake lines. Residual pressure valves are used on drum brake systems to maintain brake fluid pressure and prevent air entry when the brakes are off. Modern vehicles have tandem master cylinders to ensure braking ability in at least one circuit despite a leak. Differential pressure switches monitor loss of pressure between the hydraulic circuits. Braking units can be split front-to-rear, diagonally, or in an L shape. Diagonal and L-shaped braking splits retain 50% braking capability even if half the system fails. Quick take-up master cylinders work to compensate for the large running clearance maintained by lowdrag brake calipers. It can be dangerous to add brake fluid without diagnosing the reason for a low fluid level. Always ask customers questions to gather diagnostic information; try a test drive to understand what the customer is experiencing. Always try to discern the root cause of a vehicular problem. Common tools that are used to repair hydraulic brake systems are: brake bleeder wrenches, vacuum brake bleeders, pressure brake bleeders, and valve gauge sets. Bleeding the brakes removes air form the hydraulic braking system. The three most common brake bleeding methods are: manual, pressure, and vacuum (gravity is also used). Flushing the brake fluid involves bleeding out the air and replacing the old brake fluid with new. Always select the proper grade of brake fluid for the vehicle you are working on. Determining if brake fluid should be flushed can be done by: time/mileage, DVOM-galvanic reaction test, boiling point test, or test strip. DVOM-galvanic reaction test, brake fluid testers, and brake fluid test strips can all be used to check for brake fluid contamination. Manual bleeding requires the least amount of tools and is best when a small amount of bleeding is needed. Pressure bleeding requires more equipment and is best when the hydraulic system needs a full flush. After bleeding, always check that bleeder screws are properly tightened with no leaks, refill master cylinder and reinstall reservoir cup, and properly dispose of brake fluid. Brake pedal inspection includes brake pedal height, free play, and travel. Free play is the clearance between the brake pedal linkage and master cylinder piston. Brake pedal travel is the distance from its rest position to its applied height. Check the master cylinder for leaks if: the brake fluid is low in the reservoir, the brake warning light is on, or the brake pedal reserve height is too low. Inspect the master cylinder for internal leaks only if the brake pedal sinks. Bench bleed the master cylinder prior to installing it to minimize time needed to bleed the hydraulic brake system. Master cylinder pushrod length should only be adjusted if: someone changed the adjustment setting, the brake pedal linkage has been repaired or adjusted, or the power booster is being replaced. Brake lines are made of double-walled steel and coated to help resist corrosion. Damaged brake lines should be replaced not repaired; always use the correct tubing bender to avoid kinks. The two types of brake line flares are inverted double and ISO. Brake hoses transmit the brake system hydraulic pressures to the wheel units and must be of the proper length to be effective. Pinching or kinking a brake hose can cause it to fail. Inspect brake hoses for: cracks, bulges, abrasion or wear, kinks, and internal breakdown. Hydraulic braking systems use proportioning valves, metering valves, pressure differential valves, or anti-lock hydraulic control units to modify hydraulic pressure. Proportioning valves reduce brake pressure to the rear wheels and are pressure-sensitive or load-sensitive. Load-sensitive proportioning valves adjust rear brake pressure according to the weight of the vehicle’s load. Pressure-sensitive proportioning valves use a poppet piston to limit the rate of braking pressure increase to the rear brakes. Metering valves work to ensure rear brake pressure is applied before front brake pressure. The combination valve combines individually operating proportioning valves, metering valve and pressure differential valve in one unit and cannot be repaired. Brake warning lights alert drivers to: engagement of the parking brake, low brake fluid intake, and unequal pressure in the hydraulic brake system. Stop lights are mounted on the rear of a vehicle and alert other drivers that the vehicle is being braked. As of 1986, all vehicles must have a center highmount stop lamp (CHMSL) to reduce incidence of rear-end collisions. Power brake units are either vacuum assist (most common) or hydraulic assist. Vacuum boosters have single or dual diaphragms to extract power from atmospheric pressure and transmit force to the master cylinder. A 12-inch vacuum booster is capable of generating enough psi to stop a vehicle weighing thousands of pounds. A vacuum booster works off of the difference between manifold vacuum and atmospheric pressure; a difference in these pressures creates more force on the master cylinder pistons. Inspect and test power brake systems whenever the customer complains that the brakes are dragging, the brake pedal is harder to push than normal, the pedal height has changed, or the engine operation changes more than a minimal amount when the brake pedal is applied. 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