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BASIC BRAKE CONCEPTS HEAT When we talk about braking a car we re talking about energy conversion Converting the car s kinetic energy of motion into heat through friction Most of this heat is generated in the brake drums or discs to be passed off to the atmosphere but some of it is conducted through other brake parts If the brakes are applied hard enough and often enough so that heat is generated faster than it can be absorbed and dissipated by the brakes the condition known as fade happens This high temperature loss of brake effectiveness can affect safety if the equipment is marginal due to thin drums or rotors inferior brake lining material or other deficiencies High temperatures can also affect brake fluid seals and other rubber parts if they are of inferior quality FRICTION Since we depend on friction to convert the energy of motion into heat there are some things we should understand about it Friction is the resistance offered to the motion of one body rubbing on another The amount of friction generated depends on several things a Friction varies with the coefficient of friction of the materials in contact In brakes this would be the materials used for the brake linings and the drums or rotors b Friction varies with the area of the materials in contact Brake linings must be of adequate size and contact the drum or rotor over the maximum area to be effective c Friction is directly related to the amount of force pressing the two surfaces together While we need friction for braking it s worth noting that it also produces wear not only to the main braking surfaces the linings and drums or rotors but also to all moving parts in the brake system FLUIDS Afluid principle important to brake operation is that liquids such as brake fluid cannot be compressed while gases are compressible see Fig 1 Any gas such as air in a hydraulic system will compress readily as pressure increases thus reducing the amount of force that can be transmitted This is why all air must be expelled from the hydraulic system for it to do an effective job This is also why pure quality brake fluid free from elements that would vaporize at high temperatures must be used Another important function of the brake fluid is to provide lubrication and protect parts in the hydraulic system against corrosion Brake fluid must also be compatible with rubber parts in the system That s why it is important to put nothing but clean fresh quality brake fluid in the brake system 1 Chapter One Master Cylinder Training Fig 1 Fluid compressibility PISTON CONTAINER NO PRESSURE 50 LBS PRESSURE 50 LBS HYDRAULICS In automobile brakes the force used to press the friction surfaces together is generated hydraulically beginning with a foot pressing on the brake pedal When pressure is applied to a fluid in a closed system that pressure is transmitted undiminished in all directions to all parts of the system Thus if a pressure of 100 psi is generated in the master cylinder that same pressure is transmitted to all wheel cylinders or calipers regardless of how many there are or their location see Fig 2 Adefinite relationship exists between force and piston area in a closed hydraulic system If a force of 100 pounds is applied to a piston with an area of 1 square inch a hydraulic pressure of 100 psi will be generated Another piston in the same system with an area of 2 square inches will exert a force of 200 pounds There is also a fixed relationship between motion or travel and the piston area If the 1 square inch piston is moved 2 inches then the 2 square inch piston will move only 1 inch AIR AND VACUUM While air is to be avoided in the hydraulic system it is quite useful in vacuum power brake operation We live at the bottom of a sea of air that exerts a constant pressure on us At sea level under normal atmospheric conditions this air pressure is 14 7 psi If we remove part of the air pull a vacuum from a closed area on one side of a piston or diaphragm then a force will be exerted on the piston equal to pressure difference times the piston area In power brakes this force is used to boost the pressure of the push rod load in applying the brakes 2 Fig 2 Pressure in a closed system PEDAL FOOT PRESSURE BRAKE LEVER MASTER CYLINDER STEEL TUBING LINE LINKAGE PISTON A PISTON B LEFT FRONT WHEEL CYLINDER LEFT REAR WHEEL CYLINDER RIGHT FRONT WHEEL CYLINDER RIGHT REAR WHEEL CYLINDER DISTRIBUTION BLOCK DUALMASTER CYLINDERS As its name implies the dual master cylinder provides two separate and distinct pressure chambers in a single bore In the illustrations that follow the master cylinder is shown with the front chamber connected to the front brakes and the rear chamber to the rear brakes In some cases these connections may be reversed Some cars may have a diagonal system These alternate connections do not alter basic master cylinder operation however When the brake pedal is depressed force is transferred through the push rod to the master cylinder primary piston which moves forward Under normal conditions the combination of hydraulic pressure and the force of the primary piston spring moves the secondary piston forward at the same time When the pistons have moved forward so that their primary cups move past the bypass holes hydraulic pressure is built up and transmitted to the front and rear wheels see Fig 3 causing the brakes to be applied When the brakes are released fluid is forced back through the lines to the master cylinder However the master cylinder pistons return to the released position faster than fluid can fill the chamber thus tending to create a momentary vacuum To compensate for this fluid flows from the reservoirs through the compensating ports through the compensating holes in the pistons and around the primary cups Fig 4 NOTICE In some late model master cylinders the pistons do not have compensating holes Fig 5 Additional piston clearance is provided and other modifications made so that compensating flow is around the piston seal OD 3 Fig 3 Brakes applied Fig 4 Start of brake release Fig 5 Fluid compensation PRIMARY CUP VENT PORTS PUSHROD PRIMARY PISTON PRIMARY PISTON SECONDARY PISTON SECONDARY PISTON FLOW THROUGH PISTON FACE REPLENISHING HOLES REPLENISHING PORTS REPLENISHING PORT CYLINDER BORE CUP At the end of brake release return pressure in the lines is greater than that in the master cylinder chambers Fluid from the brake lines returns to the reservoirs through the bypass holes until pressure is equalized Fig 6 In case of a failure in the rear brake line or system the primary piston will move forward during brake apply but will not build up hydraulic pressure Only a negligible force is transferred to the secondary piston through the primary piston spring until the piston extension screw comes in contact with the secondary piston Fig 7 Then push rod force is transmitted directly to the secondary piston and sufficient pressure is built up to operate the front brakes If there is a failure in the front line or system both pistons will move forward when the brakes are applied as under normal conditions However due to the front line failure there is nothing to resist piston travel except the secondary piston spring This permits the primary piston to build up only negligible pressure until the secondary piston bottoms in the cylinder bore Fig 8 Then sufficient hydraulic pressure will be built up to operate the rear brakes With failure of either the front or rear system increased pedal travel will result and greater pedal force will be required Both of these effects should be noticeable to the driver but as an added safety feature a warning light switch is used in the system DUALMASTER CYLINDER CONSTRUCTION VARIATIONS The examples shown thus far have shown the cast iron master cylinder where the reservoir is integrally cast with the cylinder To reduce vehicle weight some new master cylinders have formed sheet metal or nylon reservoirs which are retained in the cylinder with rubber grommets While these cylinders operate in a similar manner as the cast iron units master cylinders with nylon reservoirs require a special fixture for pressure bleeding If the reservoir were pressurized as is done for the cast iron units there is the possibility that the reservoir could be distorted or broken 4 Fig 6 End of brake release Fig 7 Rear line failure Fig 8 Front line failure VENT PORT VENT PORT SECONDARY PISTON PRIMARY PISTON SECONDARY PISTON PISTON EXTENSION PRIMARY PISTON QUICK TAKE UPMASTER CYLINDER This master cylinder is designed for use in a diagonal split system It incorporates the functions of the standard dual master cylinder plus a warning light switch and proportioners Fig 9 This master cylinder incorporates the quick take up feature which provides a large volume of fluid to the wheel brakes at low pressure with initial brake application The low pressure fluid quickly provides the displacement requirements of the system created by the seal retracting pistons in to the front calipers and retraction of rear drum brake shoes The quick take up feature of the master cylinder operates as follows Figs 9 10 1 With the initial brake application more fluid is displaced in the primary piston low pressure chamber than in the high pressure chamber since the low pressure chamber has a larger diameter The additional fluid is forced around the OD of the primary piston lid seal into the high pressure chamber and on to the wheel brake units Since equal pressure and displacement must be maintained in both primary and secondary systems the primary piston moves a shorter distance to compensate for the larger volume of fluid moved from the low pressure area of the primary piston to the high pressure area 2 As the low pressure displacement requirements are met pressure will increase in the primary piston low pressure chamber until the spring loaded ball check valve in the quick take up valve opens This allows fluid to flow into the reservoir 3 After the quick take up phase of the cycle is completed the pistons function in the same manner as in a conventional dual master cylinder 4 With release of the brakes the master cylinder springs will return the master cylinder pistons faster than fluid can flow back through the systems This would tend to create a vacuum in both the low pressure and high pressure chambers of the pistons if proper compensation were not provided 5 Fig 9 Quick take up master cylinder PERIPHERAL HOLES QUICK TAKE UP LIP SEAL BY PASS GROOVE RESERVOIR SEAL NYLON RESERVOIR SEAL QUICK TAKE UP VALVE PRIMARY LOW PRESSURE CHAMBER PRIMARY HIGH PRESSURE CHAMBER SECONDARY HIGH PRESSURE CHAMBER SECONDARY LOW PRESSURE CHAMBER PROPORTIONER WARNING LIGHT SWITCH PRIMARY PISTON LIP SEAL BALL CHECK VALVE 5 The primary piston is compensated by fluid flowing from the reservoir through the small periphery holes of the quick take up lip seal through the compensating port and into the low and high pressure chambers of the primary piston The secondary piston is compensated by fluid flowing from the reservoir through the compensating port and low pressure chamber into the high pressure chamber 6 In a conventional dual bore master cylinder expansion and contraction of brake fluid is handled by fluid passing directly from the master cylinder bore through the bypass hole and compensating port to the reservoir The secondary piston in the quick take up master cylinder functions in this same manner However the primary piston must work through the quick take up valve thus a bypass groove is used to account for the fluid flow from or to the primary piston chambers 6 Fig 9 Quick take up operation A LOW PRESSURE APPLICATION B APPLICATION ABOVE TRANSITION PRESSURE C BRAKE RELEASE
