NAPA - National Automotive Parts Association Certified Mechanic Brake System Service Questions and Answers — Questions and Answers
Question 1: A customer complains of a pulsating brake pedal when stopping. Which of the following is the MOST likely cause?
- Air in the hydraulic lines
- Worn or unevenly worn brake pads
- Seized caliper piston
- Excessive rotor runout or thickness variation (Correct answer)
Correct answer: Excessive rotor runout or thickness variation
Excessive rotor runout (wobble) or disc thickness variation (DTV) causes the brake pads to move in and out as the rotor turns. This hydraulic pulsation is transmitted back through the caliper and brake fluid to the brake pedal, which the driver feels as a pulsation.
Question 2: A technician is performing a brake fluid flush. What is the primary reason for periodically replacing brake fluid?
- The fluid loses its viscosity over time.
- The fluid becomes contaminated with metal particles.
- The fluid is hygroscopic and absorbs moisture, lowering its boiling point. (Correct answer)
- The fluid's lubrication properties degrade, causing caliper wear.
Correct answer: The fluid is hygroscopic and absorbs moisture, lowering its boiling point.
Brake fluid is hygroscopic, meaning it absorbs moisture from the atmosphere. This water contamination significantly lowers the fluid's boiling point. Under heavy braking, the heat generated can cause the water in the fluid to boil, creating vapor pockets. Since vapor is compressible, this leads to a spongy pedal and a dangerous loss of braking ability.
Question 3: When bleeding the brakes on most non-ABS passenger vehicles, what is the generally accepted sequence?
- Start with the wheel closest to the master cylinder and work to the furthest.
- Bleed the front wheels first, then the rear wheels.
- Start with the wheel furthest from the master cylinder and work to the closest. (Correct answer)
- The sequence does not matter as long as all four wheels are bled.
Correct answer: Start with the wheel furthest from the master cylinder and work to the closest.
The standard procedure for bleeding most brake systems is to start with the bleeder screw on the wheel furthest from the master cylinder and progressively move to the one closest. This method helps to systematically push all the air out of the longest hydraulic lines first, ensuring a more effective and complete air purge from the system.
Question 4: After installing new brake pads and rotors, a technician should perform a bedding-in procedure. What is the main purpose of this process?
- To cool down the new components rapidly.
- To ensure the caliper pistons are fully retracted.
- To clean any oils or contaminants from the new parts.
- To deposit an even layer of friction material onto the rotor surface. (Correct answer)
Correct answer: To deposit an even layer of friction material onto the rotor surface.
The bedding-in, or break-in, process involves a series of controlled stops to heat the new pads and rotors. This heat cycle transfers a thin, uniform layer of the brake pad's friction material onto the rotor's surface. This transfer layer is crucial for optimal braking performance, noise reduction, and preventing uneven deposits that can cause vibration.
Question 5: A vehicle is brought into the shop with a complaint of a "spongy" or "soft" brake pedal. Which of the following is the MOST common cause of this symptom?
- A seized brake caliper slide pin
- Contaminated brake pads
- A malfunctioning brake booster
- Air in the hydraulic brake lines (Correct answer)
Correct answer: Air in the hydraulic brake lines
Air is highly compressible, unlike brake fluid. When air becomes trapped in the hydraulic lines, pressing the brake pedal compresses the air first before applying full hydraulic pressure to the calipers. This compression of air is what causes the pedal to feel soft or spongy.
Question 6: During a brake inspection, a technician notices that the inner brake pad on one wheel is significantly more worn than the outer pad. What is the most likely cause of this uneven wear?
- Worn wheel bearing
- Sticking caliper slide pins or a seized caliper piston (Correct answer)
- Over-tightened lug nuts
- A leaking brake hose
Correct answer: Sticking caliper slide pins or a seized caliper piston
Sticking or binding caliper slide pins, or a seized caliper piston, can prevent the caliper from releasing properly after the brakes are applied. This causes constant pressure on one of the brake pads (typically the inner pad on floating calipers), leading to accelerated and uneven wear compared to the other pad.
NAPA - National Automotive Parts Association Certified Mechanic Brake System Service Questions and Answers
A customer complains of a pulsating brake pedal when stopping.
Which of the following is the MOST likely cause?