4/15/2017 PORTER AND CHESTER INSTITUTE

Brake Hydraulic Systems

Function of the Hydraulic System The primary purpose of an hydraulic system is to transfer force from the brake pedal to the brake shoes and pads. A hydraulic system can also be used to multiply force – in the same manner as a lever multiplies force.

Advantage of using hydraulics Can transfer force over long distances Can be easily routed around obstacles Can transfer force to components that move with the suspension Never needs adjustment

Most liquids are non-compressible. Pascal’s Law Most liquids are non-compressible. When pressure is applied to a liquid in and enclosed system, that pressure is distributed equally and in all directions throughout the fluid

Pressure = Force ÷ Area 100 LBS The amount of pressure is determined by the force applied to the piston, divided by the area of the piston Piston Area = 1 Square Inch 100 ÷ 1 = 100 LBS Per Square Inch

Distribution of Pressure Pressure is distributed equally and in all directions. 30 PSI 30 Pounds 30 PSI 1 Square Inch 30 PSI 30 PSI 30 PSI

Cylinder Bore and Stroke The size of the piston is generally expressed as its ‘bore’ diameter. The distance that the piston moves is called the ‘stroke’.

The relationship between area and pressure 100 LBS 100 ÷ .5 = 200 LBS Per Square Inch 1/2 Square Inch A smaller piston will generate more pressure

The relationship between area and pressure 100 LBS 100 ÷ 2 = 50 LBS Per Square Inch 2 Square Inches A larger piston will generate less pressure

Using hydraulic pressure to transmit force Master Cylinder Slave Cylinder Hydraulic Line

If Bores of Master and Slave Cylinders are the same If the bore of the master cylinder is the same as the bore of the slave cylinder then the amount of force generated at the slave is the same as the force applied to the master cylinder. The stroke of the slave cylinder will be the same as the same as the stroke of the master cylinder.

Master and Slave with Same Size Bores Master Cylinder Slave Cylinder 100 Pounds 100 Pounds 1 inch 1 inch Hydraulic Line

Hydraulic Advantage If the master cylinder is smaller than the slave cylinder there will be a multiplication of force There will be more force output at the slave than input at the master The stroke that the master cylinder piston moves through will be longer The ratio of the area of the slave cylinder bore ÷ the area of the master cylinder bore determines the hydraulic advantage

If Master and Slave have Different Bore Diameters Master Cylinder Slave Cylinder 100 Pounds 400 Pounds 1 inch ¼ inch Bore = 1 Sq. In. Bore = 4 Sq. In.

Using Hydraulic Pressure to Transmit Force Flexible rubber hose A hydraulic master and slave cylinder can transfer force over long distances without loss of power. It can also transmit force through sharp angles and around obstacles easily It can easily connect components that are vibrating or rocking - as in a clutch linkage.

Transferring force over a distance A hydraulic master and slave cylinder can transfer force over long distances without loss of power. It can also transmit force through sharp angles and around obstacles easily It can easily connect components that are moving slightly as in a clutch linkage or suspension system.

Force Multiplication Using Leverage 1 square inch cylinder bore 50 Pounds at the brake pedal 2” Pivot Pin 8” Clevis Pin 200 Pounds at the master cylinder piston 200 PSI 200 PSI A mechanical advantage of 4 to 1 is the result of the ratio of the length between the brake pedal and pivot point divided by the distance from the pivot point to the clevis pin

Force Multiplication Using Hydraulic Advantage 300 lbs. 800 lbs. 300 lbs. Pivot Pin Caliper bore = 4 square Inch Wheel cylinder bore = 1.5 square Inch 200 PSI 200 PSI 300 lbs. The amount of pressure at the brakes is equal to the size of the cylinder bores multiplied by the pressure 800 lbs. 300 lbs.

Brake Fluid

Properties of Brake Fluid Non-Compressible Very low freezing point Very high boiling point Non-Corrosive Compatible with rubber used in brake system Good lubricant Hygroscopic [absorbs water] Compatibility with other brands and types of brake fluid

DOT Sets Standards The US Department of Transportation [DOT] sets specifications and standards for brake fluid The SAE [Society of Automotive Engineers] also sets standards

DOT designation is based on boiling temperature DOT 3 – Most Common Type in use today DOT 4 – Higher Boiling Point than DOT 3 Often required by European car manufacturers DOT 5 – Highest Boiling Point Available Used for racing and super high performance vehicles

DOT Fluid Boiling Point Dry Boiling Point 401 446 500 Wet Boiling Point 284 311 356 DOT 3 DOT 4 DOT 5

Why is Boiling Point Important? When DOT 3 fluid becomes fully saturated with moisture its boiling point drops 1170 F When brake fluid boils it forms gas bubbles The gas bubbles are compressible – just like air If the brake fluid is allowed to boil the pedal will go to the floor and the vehicle will not slow or stop!

Why Does Brake Fluid Get Saturated With Moisture The flexible rubber hoses that connect the master cylinder to the calipers and drums allow a small amount of moisture into the system through Osmosis Whenever the fluid filler cap is removed for service the fluid is exposed to the moisture in the air

POLY-GLYCOL BASED FLUID Made of Glycol-Ether compounds Made from vegetable oil not from petroleum Very hygroscopic – absorbs moisture Is used for both DOT 3 and DOT 4 fluids Additives are used in both DOT 3 and DOT 4 fluid to neutralize water held in suspension Will dissolve or discolor the paint on most cars!

LMA TYPE BRAKE FLUID ‘LOW MOISTURE ACTIVITY’ Recommended by many European car manufactures Additives in fluid help prevent deterioration of rubber seals in hydraulic system made from natural rubber compounds Manufactured by Castrol as a DOT 4 type fluid

SILICON BASED FLUID Very high boiling point Marketed in the 1980’s as a DOT 5 fluid Not hygroscopic – does not mix with water at all Tends to aerate when cycled rapidly This property makes silicone based fluid unacceptable for modern cars with ABS brake systems! Has no harmful effect on painted surfaces

DOT 5.1 Fluid Recently brake fluid manufactures have been able to formulate a poly-glycol based fluid that meets the requirements for DOT 5 fluids. Since DOT 5 has been associated with silicone based fluid this non silicone fluid is referred to as DOT 5.1

What Type of Fluid Should Be Used Most modern cars have the fluid type printed on the master cylinder filler cap

Handling Brake Fluid Brake fluid must always be kept in a sealed container If the cap is left off a container of brake fluid for 24 hours or more it should be discarded Never use an open container of fluid that you don’t recognize or remember where it came from

Handling Brake Fluid Brake fluid is clear or slightly amber in color – just like many other fluids – just because its in a container that says its brake fluid doesn’t necessarily mean it actually is brake fluid or is not contaminated with other fluids The risk of injury to people and property is too great to justify saving a few cents

Handling Brake Fluid All new containers of brake fluid have a seal under the cap that must be broken before use If the seal is broken and you are not sure how old the fluid is – discard it ! Never transfer new brake fluid into another container

Handling Brake Fluid Do not allow brake fluid to come into contact with the painted surfaces of the car body If you accidentally get brake fluid on the paint work of a car – rinse it off with a garden hose – do not wipe it off with a rag ©2005 PORTER AND CHESTER INSTITUTE / CONNECTICUT SCHOOL OF ELECTRONICS

Handling Brake Fluid Brake fluid should be changed every 2 years or 30 k miles Brake fluid turns grey and then black as the rubber components of the brake system deteriorate If you get brake fluid on you skin rinse it off immediately If you get brake fluid in you eyes – thoroughly rinse your eyes with clean water – see a physician immediately!

Petroleum Based Fluids & Brakes If a petroleum based fluid; motor oil, transmission fluid, power steering fluid – is accidentally introduced into the brake hydraulic system the rubber components will be destroyed. All of the brake hydraulic components must be replaced – the steel lines can be flushed out with alcohol

Petroleum Based Fluids & Brakes A sure sign of fluid contamination is distortion of the rubber seal on the master cylinder filler cap If this seal appears to be melted, stretched, swollen or otherwise distorted the vehicle is unsafe to drive A complete overhaul of the brake hydraulic system will be required

Master Cylinders

Single Piston Master Cylinder Used prior to 1968 Vent Port Replenishing Port Fluid Reservoir Single Spool Type Piston High Pressure Lip Seal Low Pressure Lip Seal

Single Piston Master Cylinder When Brake Pedal is Depressed Piston is Moved Forward by Brake Pedal Lip Seal Closes Vent Port Pressure Rises in Chamber Pressurized Brake Fluid is Sent to Wheel Cylinders

Master Cylinder Operation When the lip seal passes over the vent port pressure builds up in the chamber

Pumping the Brake Pedal When the brake pedal is cycle rapidly fluid can be drawn into the working chamber through holes drilled in the piston land and around the lip seal

Single Piston Hydraulic System One piston / cylinder provides all the fluid pressure for all 4 brakes

Single Piston Hydraulic System When the brake pedal is depressed fluid is distributed evenly to all 4 wheel cylinders

Brake Failure in a Single Piston System If any component in this system fails the entire brake system is inoperative

Dual Piston Master Cylinder All Modern Cars and Trucks built after 1968 use a Dual Circuit Master Cylinder

Master Cylinder Components Reservoir Primary Piston Cylinder Body Secondary Piston Return Springs

Dual Hydraulic Circuits Having two separate hydraulic circuits insures that we will have at least two working brakes in the event of a failure

Dual Hydraulic Circuits Having two separate hydraulic circuits insures that we will have at least two working brakes in the event of a failure

Dual Hydraulic Circuits If one of the rear wheel cylinders has a leak there will still be adequate hydraulic pressure to stop the car with the front brakes

Master Cylinder Operation Primary Chamber Secondary Chamber In normal braking pressure developed in the primary chamber is applied to the secondary piston The pressure on the rear of the secondary piston is transferred into the secondary chamber

Primary Circuit Failure Extension on the primary piston makes contact with secondary piston If the primary circuit fails [leaks] the primary piston will until forward it makes contact with the secondary piston

Secondary Circuit Failure If the secondary circuit fails the secondary piston will bottom out in the cylinder bore The primary piston will still be able to develop pressure

Vent Ports Vent Ports {Compensating Ports} allow fluid to flow into the chamber when the brake pedal is not depressed Fluid returns to the reservoir through the vent port when the brakes are released

Replenishing Ports The replenishing port allows fluid from the reservoir to fill the low pressure chamber behind the front piston land The replenishing port is always open to the reservoir

Quick Take Up Type Master Cylinder Provides better fuel economy by reducing brake drag

Quick Take Up Type Master Cylinder Introduced in 1980 on GM X-body Adopted by many manufactures since Used to reduce rolling drag caused by contact between pad and rotor Special design caliper pulls brake pad slightly away from rotor when brakes are not applied Additional fluid volume is needed to compensate for the fluid that is displaced by pad pull back

Small forward bore for high pressure in normal braking Step Bore Design Larger rear bore – for additional fluid displacement of quick take up calipers Small forward bore for high pressure in normal braking

2 Stage Operation Initial ‘take up’ stage Extra volume of rear section of cylinder is pushed over and around lip seals and enters both operating chambers ‘Take up volume’ pushes pads out into contact with rotor Until pads come into contact with rotor pressure in system is very low

Take-Up Stage When the brakes are initially applied the extra volume of fluid in the rear chamber passes around the lip seals of the primary piston and into the primary chamber

Pressure Stage Once pads contact rotor pressure rapidly builds up Fluid pressure in large rear section of master cylinder is allowed to vent back to reservoir through ‘quick take up valve

Quick Take-up Valve opens Take-Up Stage When the pads come into contact with the rotors pressure builds up The quick take up valve opens to allow fluid in the rear chamber to vent to the reservoir Quick Take-up Valve opens

Brake Circuits

Weight Applied to Tires 2250 lbs. 1250 lbs. The brake force needed to bring the vehicle to a stop is proportional to the weight applied to each set of wheels [axle]

How Weight Effects Brake Performance The more weight that is applied to a wheel - the harder the brake must work to slow that wheel Nearly all vehicles have the engine and transmission located over the front axle In general the brakes at the front axle must work harder than those at the rear because they carry more of the vehicle weight

Weight Transfer During Braking Decreased weight on rear wheels Increased weight on front wheels Weight All vehicles experience a weight transfer toward the front of a vehicle as the vehicle is brought to a stop

Weight Transfer Weight transfer during deceleration puts an additional load on the front brakes. The inertia of the vehicle during braking transfers force from the rear to the front effectively decreasing the weight on the rear wheels and increasing the weight applied to the front wheels

Front Brake Bias Because of the extra weight the front wheels carry and the weight transfer to the front during braking the front brakes do more of the braking than the rear For this reason the front brakes must be larger and more effective than the rear brakes Trucks that haul heavy loads are the only type of vehicle that require rear brakes that are equal to or more effective than the front brakes

Brake Bias Brake Bias is the term given to the relative percentage of braking work done by the front and rear brakes of a vehicle in a forward stop. Brake bias is expressed in percentage on the front and rear axle [ 50/50, 60/40 etc.]

Typical Brake Bias Percentages RWD 50/50 FWD 70/30 Truck – varies with load

50 / 50 Brake Bias on RWD 50 of braking done by front brakes 50 of braking done by rear brakes Rear wheel drive vehicles have a brake bias the ranges from 50/50 to 60/40

Brake Bias for Front Drive 70% of Brake Effort done by the Front Brakes 30% of Brake Effort done by the Rear Brakes Front wheel drive cars have a front rear brake bias of 70/30 and higher

Hydraulic Circuits Because there is fundamental differences in the requirements for a rear drive brake system and a front drive brake system, two different types of hydraulic system are needed Rear drive vehicles use a front / rear split hydraulic system Front drive vehicles use a dual diagonal hydraulic system In addition to the type of hydraulic circuit used in front drive and rear drive vehicles there are difference in the types of hydraulic control valves as well

Front /Rear Split Hydraulic Circuit Used on nearly all Rear Drive cars and light trucks

Front / Rear Split Dual Circuit One chamber of the master cylinder feeds both front brake cylinders – the other chamber feeds the rear cylinders As the brake bias is nearly 50/50 if one circuit fails due to a leak the other circuit can provide nearly 50% of the normal braking effort This will allow the vehicle to stop within a reasonable distance [although not as quickly as with a fully functioning braking system]

Front / Rear Split Dual Circuit On a disc / drum system one chamber of the master cylinder will feed only the disc brakes Drum Brakes Disc Brakes Since the disc brakes require a greater volume of fluid the fluid reservoirs will have different volumes – the large reservoir will be for the front brakes the smaller one for the rear

Why a Front/Rear Split Wont work on FWD If a front / rear split were used on a FWD vehicle; then if a failure occurs in the front hydraulic circuit, the rear circuit could provide only 30% of the braking effort needed to slow the vehicle Since there is so little weight carried by the rear wheels increasing the rear braking efficiency would cause the rear wheels to lock up – thus promoting a skid

Why a Left / Right Split Wont Work If the brake system is split left / right then in the event of a failure only the brakes on one side of the vehicle would work In a panic stop the vehicle would swerve left into oncoming traffic or right into the gutter

Dual Diagonal Brake Circuit Used on all FWD vehicles

Dual Diagonal Brake Circuit The solution for front drive brake hydraulic circuit was to split the system on a diagonal Connecting one chamber of the master cylinder to the left front and right rear brakes The other chamber is connected to the right front and left rear

Dual Diagonal Brake Circuit In the event that any one hydraulic component fails there will be one front and one rear brakes still working The two working brakes will be on opposite sides of the car [Left to Right] so there will be no pull if one circuit fails ©2005 Porter and Chester Institute / Connecticut School of Electronics

Trucks and SUVs Trucks and SUVs have a significant portion of their weight located over the rear axle and will normally use front / rear split brake systems Light unit body SUVs based on FWD drivelines [Subaru, Honda CRV etc.] use dual diagonal brake systems as they have a high forward brake bias

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