Power Assist and Parking Brakes Flashcards

A technician is performing a functional test of a vacuum power brake booster. With the engine off, the pedal is pumped several times to deplete the reserve vacuum, then held firmly while the engine is started. What should the technician expect from a good booster?

Answer: The pedal should drop slightly and then hold

When the engine starts, vacuum is applied to the booster and assists the pushrod, so the held pedal drops slightly. No movement indicates no vacuum assist.

A booster check valve is removed from a vacuum booster and tested with a hand-held vacuum pump. Which result indicates a good valve?

Answer: It passes air only toward the engine side

The check valve lets air be drawn out of the booster toward the intake manifold but blocks airflow back into the booster, which preserves the reserve vacuum.

A customer complains of a hard brake pedal, and a hissing sound is heard from under the dash when the pedal is depressed. The engine also idles roughly when the brakes are applied. What is the most likely cause?

Answer: A leaking booster diaphragm

A ruptured booster diaphragm lets air leak into the vacuum side, which causes the hiss, loss of assist, and a lean condition that makes the engine idle rough.

A vehicle with a hydro-boost system has a hard brake pedal and heavy steering, and a whining noise is heard from the engine compartment. Which is the most likely cause?

Answer: A slipping or broken power steering pump drive belt

Hydro-boost uses power steering pump pressure for assist, so a slipping or failed pump drive belt reduces both steering and brake assist.

Why do many diesel-powered vehicles use an engine-driven or electric vacuum pump for the power brake booster?

Answer: Diesel engines produce too little intake manifold vacuum

Diesel engines are unthrottled, so they produce very little intake manifold vacuum, and a separate vacuum pump supplies the booster.

A vehicle has an electro-mechanical brake booster that uses an electric motor instead of vacuum. The driver reports a hard pedal and a brake warning light. What should the technician do first?

Answer: Retrieve diagnostic trouble codes and check power and ground supply

Electric boosters are controlled by a module and depend on proper voltage and ground, so scanning for codes and checking electrical supply is the correct starting point.

Before adjusting a rear parking brake cable on a vehicle with rear drum brakes, what should the technician verify first?

Answer: The rear service brakes are in good condition and properly adjusted

Parking brake cable adjustment compensates only for cable slack, so the shoes and self-adjusters must be in good order first to avoid a false adjustment.

A rear disc brake system uses a small drum-in-hat parking brake assembly inside each rotor. After replacing the parking brake shoes, how is the adjustment typically made?

Answer: By turning the star wheel adjuster through the access hole in the rotor until a slight drag is felt, then backing off

The internal shoes are adjusted with a brake spoon at the star wheel, usually through an access hole in the hat of the rotor, until slight drag exists, then backed off per specification.

A technician needs to replace the rear brake pads on a vehicle equipped with an electric parking brake motor mounted on each rear caliper. What is the correct way to retract the caliper piston?

Answer: Use a scan tool to put the system in service or maintenance mode

The electric motor must retract the piston through its spindle drive using the scan tool's service mode, and forcing the piston back can damage the motor and gear mechanism.

After the parking brake lever is released, the rear wheels still drag and the rear brakes overheat. Which is the most likely cause?

Answer: A seized or corroded parking brake cable

A cable that is corroded or seized in its housing does not return fully, so the shoes or pads stay partly applied and the rear brakes drag.