Hydraulics and Power Brakes CHAPTER 31 Hydraulics and Power Brakes

Introduction Brake units are applied using principles of hydraulics. Pressing down on the brake pedal Creates pressure in the brake fluid Transmits pressure to the brake units

Principles of Hydraulics Pascal’s law principle behind hydraulic brakes Pressure = force per unit area The same pressure applied over different-sized surface areas will produce different levels of force

Hydraulic Components (1 of 8) Brake fluid has precise amount of braking force at each wheel. Three variables to consider: Input force Working pressure Output force

Hydraulic Components (2 of 8) Brake fluids graded against the United States Department of Transportation (DOT) standards: pH value Viscosity Resistance to oxidation Stability Boiling point

Hydraulic Components (3 of 8)

Hydraulic Components (4 of 8) Single-piston master cylinders have one piston with two cups (seals). Primary cup Secondary cup

Hydraulic Components (5 of 8) A single-piston master cylinder with primary and secondary cups.

Hydraulic Components (6 of 8) Recuperation: holes are drilled into piston so brake fluid can pass through inlet port, preventing vacuum.

Hydraulic Components (7 of 8) Brake fluid pressure held above atmospheric pressure by residual pressure valve Outlet end of master cylinder in single-piston master cylinder Under tube seats on tandem master cylinder

Hydraulic Components (8 of 8) Tandem cylinder has primary piston and secondary piston.

Brake Lines and Hoses (1 of 7) Brake lines and hoses carry brake fluid from master cylinder to brake units.

Brake Lines and Hoses (2 of 7) Brake lines basically same on all systems and passenger vehicles Double-walled steel Coated Attach with clips or brackets Better protect them from corrosion and physical damage in some vehicles Must conform to applicable standards such as SAE Flexible brake hose.

Brake Lines and Hoses (3 of 7) Inverted double flare First flare the end of the tube outward in a Y-shape About half the flared end is folded inside itself (inverted) Flare portion of tube is clamped between mating surfaces of two fittings Provides leak-proof connection An inverted double flared line and matching fitting.

Brake Lines and Hoses (4 of 7) Brake hose materials made of several layers of alternating materials Inside liner Liner wrapped in two or more layers Layers covered with tough flexible outer housing jacket

Brake Lines and Hoses (5 of 7) Brake hoses should never be pinched, kinked, or bent tighter than specified radius. Doing so can reduce their life cause failure of the brake hose

Brake Lines and Hoses (6 of 7) Brake hoses use fittings to connect to wheel unit. Banjo fitting Banjo bolt Copper/aluminum washers Banjo bolt assembly.

Brake Lines and Hoses (7 of 7) Brake hoses need to be checked periodically for damage or defects. Cracks Bulges Abrasion/wear Kinks Internal deterioration

Hydraulic Braking System Control (1 of 4) Hydraulic braking system must be controlled accurately to maintain control of vehicle during braking. Components help maintain control. Proportioning valves Metering valves Pressure differential valves Anti-lock HCU

Hydraulic Braking System Control (2 of 4) Proportioning valves reduce brake pressure and can be pressure-sensitive or load-sensitive. Pressure-sensitive valve in master cylinder Load-sensitive valve mounted on body or axle

Hydraulic Braking System Control (3 of 4) Poppet valve held against retainer by light return spring. During heavy braking, master cylinder pressure can reach poppet valve’s crack point.

Hydraulic Braking System Control (4 of 4) Adjustable proportioning valves require a lot of trial and error to set up properly. Not recommended for most applications

Hydraulic Braking Control System Combination valve combines pressure differential valve, metering valve, and proportioning valve(s) in one unit.

Hydraulic Braking System Control Electronic brake proportioning function reduces hydraulic pressure to rear wheels under heavy braking. Similar to proportioning valve Handled electronically

Brake Warning Lights and Stop Lights (1 of 3) Red lights are used as warning devices. Two general categories: Brake warning lights Stop lights

Brake Warning Lights and Stop Lights (2 of 3) Brake warning lights are single lights located in the instrument panel. Warn drivers of conditions in braking system that need attention. Can be illuminated by four causes: Parking brake engaged Brake fluid too low Unequal pressure in brakes Prove out or proofing circuit

Brake Warning Lights and Stop Lights (3 of 3) Stop lights warn others vehicle is braking. Mounted on rear of vehicle Must conform to federal lights for brightness and location All new passenger vehicles must have a center high-mount stop lamp (CHMSL).

Power Brakes (1 of 2) Power booster/brake uses external sources of force to multiply driver’s pedal effort. Two main types: Vacuum-assisted (most common) Hydraulic-assisted

Power Brakes (2 of 2) Vacuum booster chambers separated by flexible rubber diaphragm attached to diaphragm plate. Dual-diaphragm boosters work on same principle as single-diaphragm boosters.

Hydraulic Brake Booster (1 of 2) Many vehicles are now equipped with hydraulically assisted brake boosters. uses hydraulic pressure generated by the power steering pump rather than engine vacuum. Diesel engines

Hydraulic Brake Booster (2 of 2) Hydraulic pressure generated by power steering pump. Can be completely separate components from master cylinder or integrated components with master cylinder

Electrohydraulic Braking (1 of 3) EHB system less lesson common. Replaces booster with electronically driven hydraulic pump Uses high-pressure accumulator

Electrohydraulic Braking (2 of 3) Common tools used to repair brakes. A. Brake bleeder wrenches. B. Vacuum brake bleeder. C. Pressure brake bleeder. D. Proportioning valve/metering valve gauge sets. E. Brake fluid tester.

Electrohydraulic Braking (3 of 3) Brake fluid is the life-blood of hydraulic braking system. To check if it should be flushed: Check the time/mileage Complete a digital volt-ohmmeter(DVOM)-galvanic reaction test Or a boiling point test Or do a test strip

Maintenance and Repair (1 of 5) Manual bleeding Pressure bleeding Vacuum bleeding

Maintenance and Repair (2 of 5) For all three methods: Remove old brake fluid Refill master cylinder Determine which method of bleeding to use

Maintenance and Repair (3 of 5) Flushing brake fluid is similar to brake bleeding except in addition to removing trapped air, it also replaces old brake fluid with new brake fluid. Brake pedal height, free play, and travel are critical for proper brake operation.

Maintenance and Repair (4 of 5) Inspecting master cylinder: Brake pedal sinks when brakes are applied Brake fluid is low in reservoir Brake warning light is on Brake pedal reserve height is too low

Maintenance and Repair (5 of 5) Removing and bench bleeding the master cylinder are usually only performed when the master cylinder is being replaced. Master cylinder pushrod length is critical for proper brake operation.

Diagnosis (1 of 11) Does brake pedal sink under hard or light pressure? Does vehicle pull to left or right when braking? Does wheel lock up under hard or light pressure?

Diagnosis (2 of 11) Does it happen when it’s wet, dry, always? Does it happen when it’s hot, cold, always? Does it happen at highway speeds, stop-and-go speeds, always? Does it happen when carrying a heavy load, light load, or always?

Diagnosis (3 of 11) It’s common to take vehicle for test-drive to determine cause. All power brake systems should be inspected and tested for: Brakes dragging Pedal hard to push Pedal height changed Engine operation changes

Diagnosis (4 of 11) Vacuum boosters should be tested if vehicle has unlocated vacuum leak. Brake pedal free travel Performance/operation test External leak test Internal leak test

Diagnosis (5 of 11) Brake pedal free travel is critical for proper brake operation. Tested with brake pedal free travel test Followed by performance test

Diagnosis (6 of 11) Brake booster must have adequate vacuum to operate properly. Vacuum leaks in power boosters require increased driver foot pressure to activate brakes.

Diagnosis (7 of 11) Most issues in hydraulically assisted power brake system can be verified with visual inspection. Regular inspection of brake lines, hoses, and hardware is recommended by all manufacturers.

Diagnosis (8 of 11) To fabricate brake lines, decide which method you will use. Double flare ISO

Diagnosis (9 of 11) A mechanical brake warning lamp system (non-CAN-bus) circuit is a simple light bulb in series with as many as four switches that are connected in parallel with each other.

Diagnosis (10 of 11) In a CAN-bus circuit, the parking brake sensor and the low brake fluid sensors send signals over the network regarding their status. Diagnosis involves: Canning tool capable of interrogating the system Wiring diagram DVOM

Diagnosis (11 of 11) Diagnosis to stop lights starts with operating stop lights and observing their reaction. Best way to test is to connect a pressure gauge set designed for the high pressures within a brake system to output of the valve.

Summary (1 of 23) 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.

Summary (2 of 23) 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 cylinders.

Summary (3 of 23) 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.

Summary (4 of 23) 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.

Summary (5 of 23) 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.

Summary (6 of 23) 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.

Summary (7 of 23) Quick take-up master cylinders work to compensate for the large running clearance maintained by low-drag 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.

Summary (8 of 23) 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.

Summary (9 of 23) 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. Always select the proper grade of brake fluid for the vehicle you are working on.

Summary (10 of 23) Determining if brake fluid should be flushed can be done by: time/mileage, or doing a 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.

Summary (11 of 23) 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.

Summary (12 of 23) 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.

Summary (13 of 23) 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.

Summary (14 of 23) 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.

Summary (15 of 23) 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.

Summary (16 of 23) 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.

Summary (17 of 23) 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.

Summary (18 of 23) 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.

Summary (19 of 23) 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.

Summary (20 of 23) 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.

Summary (21 of 23) 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.

Summary (22 of 23) 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.

Summary (23 of 23) 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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