Introduction to Power Brakes Vacuum type Power Brakes
Power brakes • Drum brake systems [drum front and rear] do not require a power booster since drum brakes are self energized. • Disc brakes are not self energized and will generally require a power booster. • Lightweight sports cars can be made with 4 wheel disc brakes without a power booster but the pedal effort required to operate the brakes is alarming to most drivers.
Power booster function • The power booster augments the force applied to the master cylinder. • If the driver applies 20 pounds of force to the brake pedal and the mechanical advantage of the pedal is 5 to 1 then there will be 100 pounds of force applied to the booster. • The brake booster will generally multiply the amount of force applied to the master cylinder by a factor between 2 and 3, so that in our example where 100 pounds of force was applied to the booster- the booster will in turn apply 200 to 300 pounds of force to the master cylinder.
Power booster requirements • It must not inhibit the operation of the normal brake system in the event of a failure of the power booster. • Braking action must be proportional to the force applied to the pedal. • A fail safe reserve of at least 1 power assist application should be available in the event the engine stalls.
Types of power booster systems • Vacuum • Hydro-boost • Electric/hydraulic
Vacuum power booster system • Uses manifold vacuum as a source of power. • There is very little parasitic loss of engine power – using vacuum as a source of power draws some power from the engine at idle – but only momentarily, when the driver takes his foot off of the brake pedal. • Inexpensive & reliable - the most common system in use today.
Vacuum boosters and diesel engines • The vacuum pump is typically a vane type pump and is usually driven off of the crankshaft by a v-belt or multi-ribbed serpentine belt. • The discharge air from the vacuum pump makes a lot of noise so air output is often diverted into the air filter as a method of silencing. • Diesel powered vehicles require a vacuum pump if a vacuum power booster is to be used. Diesel engines do not produce any vacuum in the intake manifold.
Hydro-boost • Uses hydraulic pressure from the power steering pump for assist pressure. • Used primarily on diesel cars and light trucks. It is sometimes used on gasoline powered luxury cars. • A pressure accumulator provides a few ounces of fluid under very high pressure as a reserve in the event the engine stalls or the pump drive belt fails.
Hydro-boost • The booster is located in series – between the power steering pump and the steering rack.
Electric/ hydraulic • In an electric-hydraulic system the power booster piston and cylinder are integrated with the master cylinder. • Uses pressurized brake fluid to provide the boost force. • Electric pump mounted under the master cylinder or in the ABS valve assembly charges a fluid pressure accumulator with brake fluid at very high pressure. • Control valve inside the master cylinder regulates the pressure applied to the booster piston in proportion to the force applied to the brake pedal.
Electric/ hydraulic • Will work on any type of engine [gas, diesel or electric] • Since it is powered by electricity it will work when the engine is not running, making it ideal for hybrids and electric vehicles. • Has a reserve fluid pressure capacity for several applications of the brake in the event of an electrical failure. • Commonly integrated with ABS, ASC [Automatic Stability Control] and TCS [Traction Control System] braking systems.
Electric/ hydraulic • Powermaster system used on older vehicles Image of modern electro-mechanical Booster/cylinder needed
Vacuum suspended power booster system • The most common type of power booster. Used by the majority of cars and light trucks on the road today.
Vacuum suspended power booster system • The power booster uses the pressure difference between atmospheric pressure [15 psi absolute] and the air pressure in the intake manifold [5 psi absolute – when the engine is idling]. • The typical vacuum booster has a diaphragm with about 100 square inches of surface area. • With a pressure differential of 10 psi a force of 1000 pounds can be developed.
The booster consists of : • A steel chamber with a rubber diaphragm in the center • The front chamber is connected to the intake manifold by a rubber hose • The rear chamber connects to atmospheric pressure via a control valve • The diaphragm separates the booster into two chambers • The control valve opens a passage between the rear chamber and outside atmospheric pressure
Power booster construction • Stamped steel housing – clamped or crimped together at the center • Metal plate forward of the rubber diaphragm helps to reinforce the diaphragm - moves with the diaphragm • Control valve [in the center of the diaphragm] allows air to pass from the rear chamber into the front • Control valve also allows air to enter the rear chamber from a port forward of the brake pedal assembly [inside the passenger compartment]
Check valve • A check valve [one way flow valve] allows air to move from the power booster to the intake manifold but not in the opposite direction. • The check valve function is to retain vacuum in booster while vehicle is driven under full throttle. • If the engine stalls the check valve will hold vacuum in the booster to provide sufficient vacuum for one application of the brakes after the engine stalls. • The check valve is connected to the booster front chamber by a rubber grommet.
Connection to intake manifold • Rubber hose between intake manifold and booster internally reinforced with spiral steel wire to prevent hose collapse. • Never replace this hose with heater hose – heater hose will collapse under vacuum. • This hose typically has a I.D. of 9/16” to 5/8”. • A charcoal filter is sometimes located between the intake manifold and booster – the filter prevents gasoline vapors from the manifold damaging the internal parts of the booster and reduces evaporative emissions.
Vacuum booster assembly • Mounted between brake pedal assembly • Is usually attached to the firewall by four studs and nuts • The booster operating rod [input] is connected to the brake pedal by a clevis pin • A pushrod attached to the diaphragm connects the diaphragm to the master cylinder rear piston • A small clearance of a few thousandths of an inch is needed between the piston and pushrod
Booster unit Air filter • The booster unit is bolted to the firewall and is attached to the brake pedal by a yoke and clevis pin.
Air inlet and filter • Air enters the power booster in the gap between the actuator rod and the hole in the rear housing for the actuator rod. • An air filter is located at back of the rear housing. The filter prevents dirt from entering the booster and silences the rush of air entering the booster each time the brake pedal is depressed. • If you sit in the car with the engine off you will notice a slight hiss each time you press the brake pedal. • After 2 or 3 applications of the brake the hiss will stop as the vacuum in the front chamber has been bled off each time you pressed the pedal.
Vacuum booster assembly Check Valve Push Rod Reaction Spring Air Filter Adjuster Nut Control Valve Vacuum Seal Connecting Passage
Vacuum Booster operation • When the driver’s foot is not on the brake pedal there is manifold vacuum [5 psi absolute] on both sides of the booster diaphragm • Since there is always vacuum in the front chamber a fail-safe reserve of boost pressure is available in the event that the engine stalls [1 application of the brake pedal only]
Control Valve • Control valve requires a spring or rubber disk for a reaction device • As the reaction device is compressed the control valve opens the inlet passage that allows air to enter the rear chamber • When there is no pressure on the pedal the air can pass from the rear chamber to the front through a passage in the control valve. • With no pressure on the pedal the port between the rear chamber and atmospheric pressure is closed. • When the driver applies his / her foot to the brake pedal the spring in the control valve is compressed – opening the port between the rear chamber and atmosphere and closing the passage between the front and rear chambers.
Operation • Since atmospheric pressure is normally around 15 psi [absolute] a pressure differential now exist between the front and rear chambers • If the brake pedal is pressed hard enough to fully open the control valve the pressure in the rear chamber will rise to full atmospheric pressure [ 15 psia ] • If the diaphragm has a surface area of 100 square inches - the force applied to the master cylinder will be: • 10 psi x 100 sq. In. = 1000 lb. • A pressure differential of 10 pounds per square inch will be developed.
Booster operation – brakes not applied 5 psia 5 psia Check valve allows manifold vacuum to draw air out of the front chamber Control valve closes – air pressure is blocked and cannot enter rear chamber Clearance between pushrod and rear master cylinder piston allows return springs to push pistons rearward – allowing hydraulic fluid to flow back to the reservoir Passage between front and rear chambers is open – allowing vacuum to be applied to both sides of the diaphragm
Booster operation – brakes applied 5 psia 15 psia Check valve allows manifold vacuum to draw air out of the front chamber Control valve opens allowing air pressure to enter the rear chamber Control valve closes the passage between front and rear chambers. This allows a pressure differential between the font and rear sides of the diaphragm
Total force applied to master cylinder • The power assist force is in addition to the pedal effort force - so that if the driver is applying 500 pounds of force to the brake booster input - 1500 total pounds of force can be exerted on the master cylinder. • A typical brake pedal assembly will have a 4 to 1 mechanical advantage. 3” 12” 12” = 4 to 1 3”
Normal braking • During normal stopping the driver will not exert enough pressure on the brake pedal to fully open the control valve.. • Some air pressure will enter the rear chamber - but it will be less than atmospheric pressure - a pressure differential will be produced between the front and rear chambers that will add to the force already applied directly by the brake pedal. • As soon as the required braking force is achieved the driver lets off of the brake pedal slightly and the control valve goes into a hold condition where the pressure differential remains constant.
Reduced braking • When pressure applied to the brake pedal is reduced- the port between the front and rear chamber opens slightly – while the atmospheric port between the rear chamber and outside atmospheric pressure is held closed. • This reduces air pressure in the rear chamber. • The pressure differential is therefore reduced. • There now is less force applied to the master cylinder.
Pedal completely released • When the driver removes his/her foot from the brake pedal the port between the front and rear chamber is opened fully. • Air pressure between front and rear chambers equalizes at approx 5 psi absolute. • No pressure differential exists. • There is no longer any force being applied to the master cylinder pistons.
Panic stop • If the area of the diaphragm is 100 square inches then a maximum of 1000 pounds of boost pressure is available for boost assist. • Once the control valve is fully opened there can be no additional boost force. Aany addition force needed to stop the vehicle can come only from the force applied to the pedal. • If additional braking force is needed it must be provided by the drivers leg muscles.
Tandem [Dual diaphragm] power boosters • In order to get more surface area for atmospheric pressure to operate against – but still allow the booster to fit under the hood tandem diaphragm boosters were developed. • Tandem boosters are essential two boosters in series. • The housing has 4 chambers, 2 diaphragms and a center partition. • The control valve assembly in the center has passages that connect both front [manifold vacuum] chambers together and both rear [atmospheric pressure] chambers as well. • The power booster sticks out farther into the engine compartment but is not as wide as a single diaphragm unit.
Atmospheric suspended vacuum booster • Some older vehicles use a vacuum booster that has atmospheric pressure on both sides when the drivers foot is off of the brake pedal • When the driver presses the brake pedal the control valve opens - allowing manifold vacuum to pull the air out of the front chamber - • This reduces the pressure in the front chamber - resulting in a pressure differential between the front and rear of the diaphragm • When the driver takes his/her foot off of the brake pedal the control valve closes the passage to manifold vacuum and simultaneously opens up to atmospheric pressure
Disadvantage of atmospheric suspended booster • The principle disadvantage of the atmospheric suspended system is that there is no reserve of manifold vacuum to draw upon in the event that the engine stalls • Also while the vehicle is driven at full throttle there will be no power assist • This is not much of a problem - there are few instances where the brakes and throttle are applied simultaneously [stall testing an automatic transmission is one exception]
Vacuum booster sensor • Many modern cars monitor the vacuum in the booster by utilizing a vacuum sensor that connects to the booster front chamber. • Used primary on vehicles that have cylinder deactivation strategies for improved fuel economy. • After driving for extended periods with cylinders deactivated there may not be enough vacuum in the booster for proper power assist. The PCM will reactivate the cylinders for a few seconds until vacuum is restored. • The sensor is nearly identical in construction and operation to a MAP sensor. • The sensor provides diagnostic information to the PCM and ABS module via the vehicle’s CAN network.
Diagnosis • With the engine off pump the brake pedal several times - the pedal should get harder or appear to rise • While holding moderate pressure on the brake pedal - start the engine. • The pedal should drop about 1/2 to 1 inch. • If the pedal does not drop the power booster may be defective - or the vacuum hose to the power booster may be disconnected.
Air leaks • If air is heard hissing from the area directly in front of the brake pedal - where the pushrod enters the power booster - the diaphragm is probably ruptured or the control valve may be faulty. • Vacuum leaks can be detected by using an automotive stethoscope – mechanical or electronic.
Testing the check valve • With the engine shut off remove the check valve from it’s grommet on the booster housing. • A hiss should be heard as it is pulled loose. • If no hiss is heard - the check valve may be faulty or the hose may be disconnected from the intake manifold.
Testing the check valve • Hold your thumb over the check valve inlet [booster side] with the engine running to check for vacuum. • Try blowing through it – you should be able to blow from the booster side to the manifold side but not in the opposite direction.
Pedal drop test • With The Engine Off, Pump The Brake Pedal Several Times. The Pedal Should Get Harder Or Appear To Rise • While Holding Moderate Pressure On The Brake Pedal Start The Engine. The Pedal Should Drop About ½ To 1 Inch. If The Pedal Does Not Drop the Power Booster May Be Defective Or The Vacuum Hose To The Power Booster May Be Disconnected.
Pushrod adjustment • The pushrod between the diaphragm and the master cylinder can be adjusted. • A special fixture is required to make this adjustment. • Warning !!! - attempting to adjust the master cylinder without the proper tools and know how can lead to serious brake system failures. • If there is insufficient clearance between the pushrod and master cylinder the compensating ports inside the master will never open - this will allow the hydraulic pressure in the brake lines to build and never release. This may cause brake drag, premature shoe and pad failure, brake overheating, poor fuel economy and the reduction of brake effectiveness.
Pushrod adjustment check • In some cases the manufacture’s procedure for push rod adjustment check can be performed with a straight edge and caliper.
Control valve failure • Failure Mode #1 – Abnormally high pedal pressure is needed to slow the vehicle. • If the vacuum supply to the booster is good and the rest of the brake system appears to be in working order the control valve may be at fault • Failure Mode #2 – The brakes lock when the pedal is lightly depressed during normal braking. • This is an uncommon failure but one you may occasionally encounter. • In either case replacement of the booster unit is needed.
Vacuum booster replacement • R&R of the servo unit is a relatively simple process. • The servo unit can be removed with the master cylinder attached but in most cases it will be easier to remove if the master cylinder is removed first, • From inside the car the clevis pin that connects to the brake pedal is removed along with the four retaining nuts. • The check valve and hose are easily pried out of the grommet. • Since the servo unit is very large it may be necessary to remove other components to allow space for the servo to be removed.
Disassembly • On most vehicles power boosters cannot be disassembled • No internal replacement parts are available for these power booster • Some GM cars have power boosters that can be field services • Replacement diaphragms and control valves can be ordered through GM dealer parts departments
Porter and Chester Institute