ASE A5 Power Assist and Parking Brakes — Questions and Answers
Question 1: 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?
- The pedal should stay at the same height with no change
- The pedal should rise sharply against the foot
- The pedal should drop slightly and then hold (Correct answer)
- The pedal should sink slowly to the floor
Correct 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.
Pumping the pedal with the engine off removes the stored vacuum from both sides of the booster diaphragm, so the pedal feels hard. When the engine starts, manifold vacuum is applied to the booster and creates a pressure difference across the diaphragm. This pushes the master cylinder pushrod forward, and the pedal moves down slightly under the steady foot pressure. If the pedal does not move, the booster, check valve, or vacuum supply is not providing assist. A pedal that sinks steadily to the floor points to a master cylinder or hydraulic problem instead.
Question 2: A booster check valve is removed from a vacuum booster and tested with a hand-held vacuum pump. Which result indicates a good valve?
- It holds vacuum in both directions
- It passes air freely in both directions
- It passes air only toward the booster side
- It passes air only toward the engine side (Correct answer)
Correct 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.
The one-way check valve sits between the intake manifold (or vacuum pump) and the booster. It opens when manifold vacuum is stronger than the vacuum in the booster, so air leaves the booster. It closes when manifold vacuum drops, such as during hard acceleration or when the engine stops. This traps vacuum in the booster so that one or two assisted stops remain available. A valve that flows in both directions or leaks causes a hard pedal after the engine is shut off or under low-vacuum conditions.
Question 3: 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?
- A leaking booster diaphragm (Correct answer)
- A faulty brake light switch
- A restricted brake hose
- Worn rear brake shoes
Correct 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.
When the pedal is applied, the booster admits atmospheric air to one side of the diaphragm. If the diaphragm is torn, that air leaks into the vacuum side and is drawn into the intake manifold. This creates an unmetered air leak, so the engine idles roughly or stalls when the brakes are applied. The hissing noise comes from the air movement at the pedal pushrod area, and the loss of vacuum differential makes the pedal hard. The booster must be replaced, since diaphragm repair is not practical.
Question 4: 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?
- A leaking vacuum check valve
- A slipping or broken power steering pump drive belt (Correct answer)
- A faulty brake light switch
- A worn parking brake cable
Correct 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.
A hydro-boost unit uses pressurized fluid from the power steering pump to provide brake assist instead of engine vacuum. The booster and the steering gear are plumbed in series from the same pump. A slipping or broken belt, low fluid, or a weak pump reduces pressure to both, causing a hard pedal and heavy steering. Whining often points to a slipping belt or aerated fluid. The technician should check belt condition, fluid level and condition, and pump pressure.
Question 5: Why do many diesel-powered vehicles use an engine-driven or electric vacuum pump for the power brake booster?
- Diesel engines produce too little intake manifold vacuum (Correct answer)
- Diesel brake fluid requires higher pressure
- Diesel vehicles cannot use hydraulic brakes
- Diesel engines run too hot for a vacuum hose
Correct 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 gasoline engine creates vacuum behind its closed or partly closed throttle plate. A diesel engine normally has no throttle plate restricting intake air, so manifold vacuum is minimal. To provide dependable assist, a mechanical or electric vacuum pump supplies vacuum to the booster. If a vehicle with a vacuum pump has a hard pedal, the pump, its drive, and the vacuum lines should be tested with a vacuum gauge.
Question 6: 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?
- Replace the vacuum check valve
- Bleed the booster with a vacuum pump
- Retrieve diagnostic trouble codes and check power and ground supply (Correct answer)
- Install a new master cylinder immediately
Correct 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.
An electro-mechanical booster uses an electric motor and gear mechanism, commanded by a control module, to assist pedal force. Because there is no vacuum supply, the diagnosis starts with the electrical side: scan the control modules for stored codes, then verify battery condition, fuses, power and ground circuits, and connector condition. Low system voltage can limit or disable assist. Replacement of components should come only after the electrical checks and any manufacturer procedures are done.
Question 7: Before adjusting a rear parking brake cable on a vehicle with rear drum brakes, what should the technician verify first?
- The parking brake lever is fully applied
- The front brake pads are new
- The brake fluid has been flushed
- The rear service brakes are in good condition and properly adjusted (Correct answer)
Correct 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.
The parking brake applies the same rear shoes or pads as the service brakes, so excessive travel is often the result of worn linings or an inoperative self-adjuster rather than cable stretch. The technician should inspect and adjust the rear service brakes first, then release the lever and adjust the cable at the equalizer or adjuster. After adjustment, the rear wheels should turn freely with the lever released, and the brakes should hold firmly within the specified number of lever clicks.
Question 8: 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?
- By turning the star wheel adjuster through the access hole in the rotor until a slight drag is felt, then backing off (Correct answer)
- By loosening the caliper bracket bolts and rotating the caliper
- By tightening the parking brake cable until the lever is rigid
- By pressing the brake pedal repeatedly until the shoes seat
Correct 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 drum-in-hat design uses small parking brake shoes inside the center of the rotor, independent of the hydraulic caliper. After installing new shoes, the technician rotates the star wheel adjuster with a brake spoon or screwdriver through the rotor's access hole until the rotor drags slightly, then backs it off the specified number of notches. The cable is then adjusted so the shoes are not applied with the lever released. Over-tightening causes rear-wheel drag and overheating.
Question 9: 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?
- Compress the piston with a C-clamp
- Turn the piston in with a disc brake spreader tool
- Use a scan tool to put the system in service or maintenance mode (Correct answer)
- Open the bleeder and push the piston by hand
Correct 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.
On caliper-mounted electric parking brake systems, an electric motor and gear train drive a spindle that moves the piston to apply the parking brake. To service the pads, the technician activates the service or maintenance mode with a scan tool (or the manufacturer's specified procedure), which retracts the piston electrically. Forcing the piston back with a clamp can strip the gears or damage the motor. After the work, the system is typically returned to normal mode and calibrated per the service information.
Question 10: After the parking brake lever is released, the rear wheels still drag and the rear brakes overheat. Which is the most likely cause?
- A leaking booster check valve
- Low power steering fluid
- A seized or corroded parking brake cable (Correct answer)
- An overfilled brake fluid reservoir
Correct 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.
The parking brake is applied mechanically by cables that must slide freely in their housings and return under spring tension. Corrosion, a frayed strand, or a binding equalizer can keep the cable from retracting, so the rear shoes or pads remain partly engaged. This produces drag, heat, a burning smell, and premature lining wear. The technician should check cable movement with the lever released, free or replace the cable, and lubricate pivot points as appropriate before adjusting.
ASE A5 Power Assist and Parking Brakes
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?
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