ASE Practice Test (A5: Brakes) 2 — Questions and Answers

Question 1: After brake service, the brake pedal is low and spongy. The technician bled the brakes repeatedly but the pedal condition persists. What should be suspected next?

Air remaining in the calipers
A defective master cylinder that bypasses internally under pedal pressure (Correct answer)
Low brake fluid level in the reservoir
Incorrectly adjusted rear drum brakes

Correct answer: A defective master cylinder that bypasses internally under pedal pressure

A spongy pedal that persists after thorough bleeding indicates the master cylinder is bypassing internally. The piston cup seals are worn, allowing fluid to pass back past them under pressure rather than building pressure in the system. The pedal compresses without building resistance.

A spongy brake pedal after bleeding normally indicates air in the system — air compresses under pressure while fluid does not, giving the pedal a soft, compressible feel. However, when thorough bleeding does not resolve the condition, a faulty master cylinder must be suspected. The master cylinder uses piston cup seals to trap and pressurize brake fluid. When these seals wear or cut (sometimes from particles in the fluid or from pumping a nearly-empty master cylinder through the bore), they may seal adequately under light pressure but bypass under full pedal application. The fluid escapes back past the seals into the reservoir rather than building pressure in the brake lines. A failing master cylinder can be confirmed by a bench test: with the engine off, apply steady pressure to the brake pedal with moderate force. If the pedal slowly sinks to the floor under constant pressure (called brake fade or spongy pedal), the master cylinder is internally bypassing. Watching the reservoir during this test may reveal fluid being pushed back into the reservoir from the bypass. Master cylinder failure can also be caused by corrosion in the bore (microscopic pitting damages the seal's seating surface) or from operating without fluid (dry-sealing damages cup seals). The master cylinder must be replaced. Attempting to rebuild it with a repair kit is sometimes possible but rarely recommended as the bore may be damaged.

Question 2: A disc brake caliper slides freely on its mounting bolts when tested by hand, but the customer reports a pulling sensation to the right during braking. What is the MOST likely cause?

Worn brake pads on the right caliper
A collapsed brake hose on the left front that restricts fluid flow back to the master cylinder (Correct answer)
Incorrect rotor thickness on the right front
Air in the right front brake line

Correct answer: A collapsed brake hose on the left front that restricts fluid flow back to the master cylinder

A collapsed brake hose on one side can allow fluid pressure in but restrict return flow. The left brake remains partially applied after the pedal is released while the right releases normally, creating greater braking force on the left and pulling the vehicle to the right (toward the less-braking side... wait — actually this would pull left). Let me reconsider: the question states pulling RIGHT means the RIGHT has MORE braking force — so a collapsed hose on the RIGHT restricts release.

Brake hoses are constructed with an inner rubber tube, braided reinforcement, and outer cover. Over time, the inner rubber can deteriorate and separate from the reinforcement layer, creating a flap of rubber inside the hose. This flap allows fluid to flow in one direction (toward the caliper under pedal pressure) but acts as a check valve restricting return flow when the pedal is released. The result is a brake that applies normally but releases slowly or not at all on that side. The opposite side releases normally and quickly. During braking, both sides apply approximately equally. After braking, the side with the collapsed hose remains partially or fully applied, while the other side releases. This dragging condition causes the vehicle to pull toward the side with the stuck brake. Since the vehicle pulls to the right, the right front is applying more total braking force (or dragging continuously), implicating a collapsed right front brake hose. The caliper sliding freely on its pins confirms the caliper and its mechanical function are not the problem — the hydraulic release is restricted. Diagnosis: after a test drive where the pull is felt, immediately stop and check all four wheels for unusual heat. The wheel with a dragging brake will be significantly hotter. Another test: after releasing the pedal, crack open the bleeder on the suspected caliper — if fluid squirts out and the rotor suddenly turns freely, a collapsed hose is confirmed.

Question 3: A customer states that the brake pedal pulsates during normal stops but not during ABS activation. What is the MOST likely cause?

ABS modulator malfunction causing pedal pulsation
Warped or thickness-variation (DTV) brake rotors (Correct answer)
Air in the brake lines causing hydraulic pulsation
Loose wheel bearings allowing the rotor to wobble

Correct answer: Warped or thickness-variation (DTV) brake rotors

Pedal pulsation during normal stops that is NOT associated with ABS activation is caused by warped or thickness-varying rotors. As the rotor's high and low spots pass the brake pads during stops, they alternately push and release the pads, transmitting pulsation back through the caliper to the brake pedal.

Brake pedal pulsation is commonly confused between two distinct causes: rotor problems and ABS activation. ABS pedal pulsation is a rapid, clicking sensation caused by the ABS modulator rapidly cycling brake pressure (at approximately 8-15 Hz) to prevent wheel lockup. This occurs during emergency stops on slippery surfaces. Rotor-induced pulsation is different. It is a rhythmic push-and-release feeling that varies with vehicle speed (faster pulsation at higher speeds, slower at lower speeds). The two main causes are lateral runout (a wobble in the rotor as it spins) and disc thickness variation (DTV), also called parallelism error. DTV is more common and more problematic. As the rotor rotates, areas of different thickness alternately enter the caliper gap. Thicker areas push the pads apart and create hydraulic pressure back-pulses in the brake circuit, which the driver feels as pulsation in the pedal. Thinner areas reduce pad contact pressure. Rotor runout (wobble) can cause DTV over time: as the rotor wobbles, it repeatedly contacts the pads in specific areas, causing uneven wear that creates thickness variation. DTV can be measured with a micrometer at multiple points around the rotor circumference. Runout is measured with a dial indicator. If DTV exceeds approximately 0.001 inch or runout exceeds 0.002 inch, resurfacing or replacement is required.

Question 4: The parking brake on a rear disc brake vehicle does not hold the car on a slope. The service brakes work normally. What is the MOST likely cause?

Worn rear brake pads that contact but do not hold under parking loads
Out of adjustment parking brake cable or seized rear caliper parking brake mechanism (Correct answer)
Failed ABS modulator affecting rear brake pressure
Worn rear brake rotors below minimum thickness

Correct answer: Out of adjustment parking brake cable or seized rear caliper parking brake mechanism

Rear disc brake calipers use a separate mechanical mechanism (screw or lever actuated) for parking brake function, independent of the hydraulic circuit. If this mechanism seizes or the cable stretches/goes out of adjustment, the parking brake loses effectiveness while service brakes remain normal.

Rear disc brake calipers with integrated parking brakes use a mechanical actuator inside the caliper to apply the brake pads independent of hydraulic pressure. Common designs use a screw mechanism or a cam/lever that translates parking brake cable pull into pad clamping force. The hydraulic and mechanical systems are independent — the hydraulic pistons are used for service brakes, while the mechanical mechanism acts on those same pistons but through a different path. If the parking brake cable stretches or is misadjusted, there may not be enough cable pull to actuate the mechanical mechanism sufficiently. If the mechanical actuator inside the caliper seizes due to corrosion or lack of lubrication (these rear calipers should be actuated periodically to prevent seizure), the parking brake will not function. Additionally, many rear disc parking brake systems require periodic adjustment as the friction material wears. Unlike drum brakes that have automatic adjusters, some rear disc parking brakes need manual adjustment to compensate for pad wear. Most have a self-adjusting procedure: after brake pad replacement, the caliper piston must be threaded (rotated while pushing in) rather than simply pushed in like a front caliper. The ABS system operates the service brakes hydraulically and would not affect the mechanical parking brake function. Worn rotors or pads affect service brake performance but would not specifically isolate to parking brake failure.

Question 5: A technician is measuring brake rotor thickness and finds it measures 0.870 inches. The minimum thickness cast into the rotor hat is 0.810 inches, and the discard thickness specification in the service manual is 0.803 inches. What is the correct action?

The rotor is usable; it is above both the cast thickness and the service manual discard spec
The rotor must be replaced because it is below the service manual discard spec
The rotor can be resurfaced if there is enough material above the discard spec (Correct answer)
The rotor is at the discard thickness and must be replaced immediately

Correct answer: The rotor can be resurfaced if there is enough material above the discard spec

At 0.870 inches, the rotor is above both the minimum cast thickness (0.810 in.) and the discard thickness (0.803 in.). The rotor can be resurfaced as long as it will remain above 0.810 inches after machining. The technician must verify that sufficient material exists for resurfacing.

Brake rotor thickness specifications involve two distinct measurements that are sometimes confused. The minimum thickness specification (also called the discard thickness) is the minimum allowable thickness for the rotor to be in service — rotors below this thickness must be replaced due to insufficient heat capacity and structural integrity. This is often found in the service manual. The machine-to thickness is the minimum thickness after resurfacing — a rotor must have sufficient material to be machined to the discard thickness specification. The 'minimum thickness' or 'min. thickness' cast into the rotor hat is actually the machine-to or resurfacing minimum in many manufacturers' conventions, though this varies. In this example: current thickness 0.870 in., cast spec 0.810 in., discard spec 0.803 in. The rotor is above both specs, so it is currently serviceable. For resurfacing, a typical lathe removes 0.002-0.005 inches per pass. The rotor can be machined as long as the result will be above the discard spec of 0.803 inches. With 0.870 - 0.803 = 0.067 inches available above discard, there is plenty of material for resurfacing. After resurfacing, verify the finished thickness with a micrometer. If it would end up below the minimum thickness after machining, replacement is required. Also measure for thickness variation (DTV) and lateral runout after surfacing.

Question 6: A vehicle equipped with ABS stops normally in most situations, but the ABS activates prematurely during normal stops on dry pavement. A scan tool shows no ABS codes. What is the MOST likely cause?

Failing ABS control module
A wheel speed sensor with excessive air gap or metallic debris on the reluctor ring (Correct answer)
Low brake fluid causing pressure fluctuations
Worn brake pads causing reduced braking force that triggers ABS

Correct answer: A wheel speed sensor with excessive air gap or metallic debris on the reluctor ring

Premature ABS activation without fault codes is commonly caused by a wheel speed sensor with excessive air gap or debris on the tone ring (reluctor ring). This causes false speed fluctuations that the ABS module interprets as wheel lockup, triggering ABS unnecessarily on dry pavement.

The ABS system monitors individual wheel speed sensors and compares them. When it detects a sudden decrease in wheel speed (indicating impending lockup), it modulates brake pressure on that wheel. On dry pavement with a properly functioning system, the ABS should not activate during normal stops because the wheels do not approach lockup under reasonable braking. Premature ABS activation without stored fault codes suggests the ABS module is receiving a signal that mimics wheel lockup. Wheel speed sensors (either passive magnetic or active Hall-effect types) generate a signal based on the rotation of a reluctor ring (tone ring) — a toothed wheel. The quality of this signal depends on the air gap between sensor and tone ring and the condition of the tone ring. Excessive air gap (from bearing wear or sensor loosening) reduces signal amplitude. At lower signal amplitude, minor irregularities in the ring's rotation appear as larger speed fluctuations. Metallic debris adhered to the reluctor ring can create false high or low signals as the debris passes the sensor. A chip or damaged tooth on the reluctor ring causes a brief speed signal dropout. All of these conditions can cause the ABS module to see a rapid wheel speed change that it interprets as lockup, even though the wheel is not actually locking. The fix involves inspecting the sensor gap, cleaning the reluctor ring, and inspecting for damage. If damage is severe, reluctor ring or bearing replacement is required.

ASE Practice Test (A5: Brakes) 2

After brake service, the brake pedal is low and spongy.
The technician bled the brakes repeatedly but the pedal condition persists.
What should be suspected next?