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Power Brake Boosters


2/1/2006
By Larry Carley

 

Where would your customers be without these amazing devices? In the emergency room? Power brake boosters are standard equipment on virtually all vehicles today thanks to disc brakes and the motoring public's desire for effortless braking. The power assist provided by the brake booster allows a 120 lb. soccer mom to bring 6,000 lbs. of Lincoln Navigator to a screeching halt with little more than a light caress of her foot against the brake pedal.

A brake booster assists braking by multiplying the effort applied to the master cylinder when the driver pushes down on the brake pedal. The booster doubles or triples the force applied. This reduces the pedal effort needed to stop the vehicle for easier, safer braking.There are three basic types of brake boosters:

  • Vacuum Boosters - These are the most common type. They use a vacuum diaphragm connected to a vacuum port on the engine's intake manifold. A vacuum booster may be used with ordinary power brakes or with many "non-integral" antilock brake systems.

  • Hydro-Boost - This type of booster uses hydraulic pressure from the power steering pump to assist braking. It may be used on vehicles with or without ABS.

  • Electro-Hydraulic - This type of power assist is used with "integral" antilock brake systems as well as some of the newer hybrid electric vehicles and those with "brake-by-wire" systems (these are brake systems that use input from a brake pedal position sensor to activate the booster and apply the brakes). The booster is part of the hydraulic module and uses a pump and high-pressure gas-charged accumulator to assist braking.

    Let's take a look at each of these three types to better understand how they work and why they might need to be replaced.

VACUUM BOOSTERS
Vacuum-actuated brake boosters are used on many vehicles because they have a relatively simple design and use intake vacuum to multiply brake force. Vacuum is the absence of atmospheric pressure, and the higher the vacuum, the greater the atmospheric push to fill the void. As the clich goes, "Nature abhors a vacuum." As soon as you create a vacuum by sucking air out of something (like the intake manifold), the surrounding air tries to rush in and fill the void. Consequently, the push provided by a vacuum booster is really atmospheric air pressure working against vacuum.

At sea level, normal air pressure is 14.7 lbs. per square inch. If you were to suck all the air out of a cylinder, seal it tightly and then measure the vacuum with a gauge, it would read about 30 inches of Mercury (Hg). By comparison, the typical engine pulls about 17 to 21 inches of vacuum at idle. As the cylinders suck air out of the intake manifold, it creates a partial vacuum. But it never achieves a full vacuum because more air keeps entering the engine through the throttle body. The engine has to have air to run, otherwise it would be nothing more than a starter-driven vacuum pump.

In diesel engines, there is no throttle to create a restriction so diesel engines never develop any vacuum at idle. As a result, diesels have to use an auxiliary vacuum pump if they have a vacuum brake booster.

How the brake booster uses vacuum to provide power assist is amazingly simple. The original "Master-Vac" power brake booster that became the predecessor to virtually all vacuum boosters today was patented back in the 1950s by Bendix. The booster housing is divided into two chambers by a flexible diaphragm. A vacuum hose from the intake manifold on the engine pulls air from both sides of the diaphragm when the engine is running. When the driver steps on the brake pedal, the input rod assembly in the booster moves forward. This blocks off the vacuum port to the backside of the diaphragm and opens an atmospheric port that allows air to enter the back chamber. As a result, the diaphragm has vacuum pulling against one side and air pressure pushing on the other. This creates the force that multiplies the force of the driver's foot on the brake pedal.

The amount of power assist that is provided by a vacuum booster depends on two things: intake vacuum and the size of the booster diaphragm. The larger the diaphragm, the greater the assist. An 8-inch booster with 20 inches of engine vacuum will provide about 240 lbs. of brake assist. As a rule, larger vehicles require more assist (and a larger vacuum booster) than smaller vehicles.

When the engine is off, vacuum is trapped in the booster housing by a one-way valve. On some vehicles, there may be a separate vacuum reservoir. There is usually enough stored vacuum for a couple of power-assisted brake applications. But once the reserve vacuum has been used up, pedal effort goes up dramatically.

If a vacuum booster fails, it can't provide normal power-assisted braking. The vehicle can still be driven, but the brake pedal will feel much stiffer and require more effort to apply the brakes. In 2000, new Federal Motor Vehicle Safety Standards (FMVSS) 135 were introduced that required 2000 model year and newer passenger cars and 2002 and newer light trucks to meet tougher stopping distance requirements if the brake booster fails. To meet these regulations, the brake systems on many of these vehicles have been upgraded with larger and/or more aggressive brake linings (which is something to keep in mind when recommending replacement linings to a customer).

WHEN GOOD BOOSTERS GO BAD
Vacuum brake boosters are very reliable and will often last the life of the vehicle. But after so many years, the diaphragm or check valve may fail causing a loss of power assist and a significant increase in the pedal effort required to stop the vehicle. The diaphragm can be adversely affected if the master cylinder is leaking and brake fluid is siphoned into the booster. The presence of brake fluid inside the booster or vacuum hose, therefore, would tell you that the master cylinder is leaking and needs to be replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.

Brake assist problems can also be caused by a loose, leaky, kinked or plugged vacuum supply hose. A restricted vacuum hose will cause boost to drop off when the brakes are applied in rapid succession. This happens because the blockage slows the return of vacuum in the booster.If the external one-way check valve on the booster's vacuum supply hose is leaking, the booster may not hold adequate vacuum when the engine is under load, causing a temporary increase in pedal effort. If the engine has low intake vacuum because of a manifold leak, exhaust blockage or mechanical problem, the vacuum booster may not be able to deliver its normal amount of assist. The problem here is not the booster but low intake vacuum at the engine.

VACUUM BOOSTER PEDAL CHECK
A simple way to check a vacuum brake booster is to pump the brake pedal with the engine off several times to bleed off any residual vacuum from the booster. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. If the pedal doesn't move, check the vacuum hose connection and engine vacuum at the intake manifold. If okay, the problem is a leaky diaphragm inside the booster and the booster needs to be replaced.

The external one-way check valve at the booster vacuum hose inlet can be checked by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.

Because the vacuum booster is mounted between the master cylinder and firewall, replacing the booster requires unbolting the master cylinder and moving it forward so the booster can be replaced. Usually, there's no need to disconnect any of the brake lines unless access is very restricted.

The pushrod that runs from the booster into the back of the master cylinder must have a specified amount of play (see a service manual for particulars). Most require a small amount of play so the master cylinder will release fully preventing brake drag, but some late-model GM and Bendix applications have zero play.

HYDRO-BOOST
Hydro-Boost was first introduced back in 1973. Hydro-Boost uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide power assisted braking.

Inside the Hydro-Boost unit, which fits between the master cylinder and brake pedal, is a spool valve and piston assembly. When the driver steps on the brake pedal, the pushrod slides forward and changes the position of the spool valve. This opens a valve port that routes power steering fluid into the cavity behind the piston to push it forward and apply the brakes.

Hydro-Boost also uses an "accumulator" to store pressure. Some accumulators are nitrogen pressurized while others are spring loaded, depending on the application. The accumulator provides backup pressure in case normal hydraulic pressure is lost (because the engine stalls or the power steering pump drive belt breaks). There's usually enough reserve pressure in the accumulator for 1 to 3 power assisted stops.

Problems with this system can be caused by spool valve or piston wear inside the Hydro-Boost unit, fluid leaks or loss of pressure due to a worn power steering pump, slipping pump belt or low power steering fluid level. Slow brake pedal return may be caused by excessive seal friction inside the booster, faulty spool action or a restriction in the return line to the pump. Grabby brakes are probably the result of contamination inside the system or a broken spool return spring. If the brakes apply by themselves with no pedal effort, the system may have restricted return flow or a defunct dump valve. Excessive pedal effort can be caused by internal leakage or the seeping of fluid past the accumulator/booster seal.

A simple way to test the Hydro-Boost system is to pump the brakes five or six times with the engine off to discharge the accumulator. Then press down hard on the pedal (about 40 lbs. of force) and start the engine. Like a vacuum booster, you should feel the pedal fall slightly, then rise when the engine starts.

The leakdown of the accumulator can be checked by pumping the brakes several times while the engine is running, then shutting it off. Let the car sit for about an hour, then try the brakes without starting the engine. You should get two or three soft brake applications before it takes more effort to push the pedal.

If a braking problem turns out to be in the booster, the booster will have to be replaced. Warning: The Hydro-Boost accumulator must be fully depressurized by pumping the brake pedal a dozen times before any hydraulic lines are opened or the booster is disconnected.

ELECTRO-HYDRAULIC
On vehicles with integral antilock brake systems where the master cylinder is part of the hydraulic control assembly (Teves Mark 2 ABS, Bosch III ABS, Delco Powermaster 3 ABS, Bendix 10 and Jeep ABS), an electric pump with a nitrogen pressurized accumulator is used to provide power assist.

On these applications, power assist is provided by pressure stored in the accumulator. We're talking lots of pressure here, from 675 to 2,600 psi depending on the system and application. When the driver steps on the brake pedal and the pushrod moves forward, it opens a valve inside the master cylinder that allows stored pressure from the accumulator to enter a cavity behind the piston assembly. This pushes the piston forward and applies the brakes.

A pressure switch on the master cylinder monitors the stored pressure in the accumulator, and closes a switch to turn on the electric pump when pressure drops below a preset minimum. It then turns the pump off when pressure is back up to where it should be.

Problems with this type of power brake system will usually be due to a bad pump motor, pump motor relay, a leaky accumulator or internal problems in the master cylinder assembly. Because it's all part of the ABS system, electrical problems with the pump motor, motor relay or pressure switch, as well as low fluid level or low pressure will usually set a diagnostic trouble code (DTC) and turn on the ABS and/or brake warning lights.

The electric pump and accumulator can usually be replaced separately if there's a problem, but the master cylinder and hydraulic control unit are combined and must be replaced as an assembly (which is very expensive!).

Here's another warning: The accumulator must be depressurized prior to opening any hydraulic lines or disconnecting the ABS master cylinder/modulator assembly. On ABS Electro-Hydraulic systems, the pedal needs to be pumped 40 times with the engine off (or until an increase in pedal effort is clearly felt) to bleed off all the pressure from the accumulator.













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