ASE Certification Guide

ASE A5 · diagnosis by symptom

Brake pedal pulsation and other symptoms

Brake pedal pulsation during a normal stop points first at the rotor: its thickness varies around the circumference, a fault that lateral runout can start by letting the pads touch one spot. On drum brakes the same feel comes from an out-of-round drum. A pulse only in a hard or slippery stop is the ABS working. This page sorts each symptom on the A5 task list by system, the way the test asks it: complaint first, part second.

  • 9symptoms sorted by system
  • 8diagnosis questions
  • 4content areas they draw on

Symptom → system → check

Pedal pulsation

  • Disc: rotor thickness variation (micrometer at several points around the rotor), lateral runout (dial indicator), loose wheel bearing, uneven lug torque. Disc brakes
  • Drum: out-of-round drum; measure it, then machine or replace. Drum brakes
  • ABS: pulsing with pump noise on a dry, slow stop is false activation, from a cracked or dirty tone ring or a bad wheel speed signal. Electronic brake control

Pedal sinks under steady pressure

First rule out an external leak at the wheels, the lines and the joint between master cylinder and booster. With none, fluid is bypassing the master cylinder cups inside the bore. Hydraulics

Low pedal, firm when pumped

Too much shoe-to-drum clearance from self-adjusters that do not work, or air in the system. Check the rear adjustment before you bleed.

Spongy pedal

Air in the hydraulic system: a master cylinder installed without bench bleeding, air trapped in the ABS hydraulic unit, or fluid that has taken on water and boils when hot. A swelling hose feels the same.

Hard pedal

Lost power assist. With the engine off, pump out the booster reserve, hold the pedal and start the engine: it should drop slightly. If it does not, check engine vacuum or the auxiliary pump, the check valve and the booster diaphragm, or the hydro-boost supply.

Pull while braking

If the car drives straight but pulls when braking, look at the brakes: a seized caliper piston or slide, a collapsed hose that holds or blocks pressure, linings soaked with grease or fluid. A pull that is also there while driving belongs to tires or alignment.

One wheel grabs

Contaminated linings, a loose caliper or a faulty valve in the ABS hydraulic unit, the last of these being a case the study guide’s samples use.

Drag that builds with each stop

Brakes that tighten after every application, and release for a while after bleeding, mean fluid cannot return past the master cylinder compensating port: wrong pushrod length or swollen cups. A single dragging wheel is more likely a caliper or hose.

Vibration, but a steady pedal

If the steering wheel shakes while braking but not while coasting, the front brakes are the source. If it shakes while coasting too, look at the tires and wheels.

A pedal that drops at engine start is normal

Hold the brake pedal, start the engine, and the pedal moves down slightly as vacuum assist builds. That is the booster test passing, not a fault. A two-technician item that blames the diaphragm or the check valve for it is asking you to say “neither”.

8 diagnosis questions

0 of 8 answered · 0 correct
Question 1 of 8 · Electronic Brake Control Systems: ABS, TCS, and ESC Diagnosis and Repair · 22%Not answered

A driver experiences a pulsating brake pedal and ABS pump motor noise only during slow, dry pavement stops. There are no diagnostic trouble codes stored in the system. What is the MOST likely cause of this condition?

Explanation

Correct: A cracked tone ring makes the wheel speed signal drop out at low speed, so the module sees a false lockup and cycles ABS (pump noise, pulsing pedal) in a slow, dry stop.

Incorrect: An open brake pedal travel switch sets a code of its own; it does not fake a locking wheel at low speed.

Incorrect: Air in the hydraulic lines makes the pedal spongy; it does not make the ABS pump run.

Incorrect: A failing pump motor would be noisy or slow whenever ABS works; it cannot trigger ABS on dry pavement by itself.

Incorrect: Warped front rotors (really thickness variation) pulse the brake pedal at any speed, but they do not run the ABS pump.

Question 2 of 8 · Hydraulic, Power Assist, and Parking Brake Systems Diagnosis and Repair · 43%Not answered

A technician replaces a leaking master cylinder and performs a conventional four-wheel brake bleed. No external leaks are found, but the brake pedal feels very spongy and goes to the floor. What is the MOST likely cause of this condition?

Explanation

Incorrect: Vacuum bleeding is an accepted method; at most it draws a few bubbles past the bleeder threads, which never reach the master cylinder.

Incorrect: Pushrod clearance changes free travel and can cause drag if too long, but it does not make the pedal spongy.

Correct: Air left in a dry master cylinder's bores is hard to push out through the lines with a normal wheel bleed; bench bleeding before installation is the step that prevents it.

Incorrect: An open bleeder screw would leak fluid at the wheel, and no leaks were found.

Incorrect: Tripping the proportioning (or pressure differential) valve while bleeding can light the warning lamp, but it does not trap air or make the pedal spongy.

Question 3 of 8 · Disc Brake Diagnosis and Repair · 24%Not answered

After track use, a vehicle has brake pedal pulsation. The rotor has visible pad imprints and measures 26.1 mm; the manufacturer's minimum thickness is 26.0 mm. What should the technician do?

Explanation

Incorrect: Scuffing the faces cleans the surface but leaves the thickness variation that causes the pulsation.

Incorrect: Uneven lug torque can distort a rotor, but re-torquing cannot remove deposits and imprints already in the friction surface.

Incorrect: Indexing moves the rotor on the hub to reduce lateral runout; it does nothing for imprints or thickness variation in the rotor itself.

Correct: Pad imprints sit in the friction surface, and any cut deep enough to remove them takes the rotor below the 26.0 mm minimum, so a new rotor is the only fix within spec.

Incorrect: Machining would remove the imprints, but with only 0.1 mm above minimum the rotor would end up under the manufacturer's limit.

Question 4 of 8 · Disc Brake Diagnosis and Repair · 24%Not answered

A vehicle has a steering wheel shimmy during light braking at 50 mph but no vibration when coasting at the same speed. The brake pedal does not pulsate. Which is the most likely cause?

Explanation

Incorrect: Glazed front brake pads reduce bite and squeal, but a smooth glazed surface does not create a rhythmic shake.

Incorrect: Unequal front tire pressure makes the car pull under braking; it does not cause a rhythmic shimmy.

Incorrect: Wheel and tire imbalance shakes the car at that speed whether the brakes are on or not, and here coasting is smooth.

Incorrect: An out-of-round rear drum causes a pulse felt in the pedal or seat, not a shimmy through the steering wheel.

Correct: A shake that appears only when the pads clamp the rotor comes from the brakes; lateral runout wobbles the caliper and feeds into the steering even before the pedal starts to pulse.

Question 5 of 8 · Disc Brake Diagnosis and Repair · 24%Not answered

A vehicle exhibits a soft brake pedal that requires pumping to achieve normal height after aggressive cornering. The brake fluid level is full and there are no visible leaks. The technician suspects pad knockback. Which of the following conditions is the most likely root cause?

Explanation

Incorrect: A restricted brake line slows apply or release and can cause pull or drag, but it does not push the pads back.

Incorrect: An internal master cylinder leak makes the pedal sink under steady pressure at any time, not only after cornering.

Incorrect: Worn pads add travel on every stop; they do not explain a pedal that is low only after hard cornering.

Correct: Excessive wheel bearing play lets the rotor wobble in hard cornering and push the pistons back (knockback), so the first pedal stroke only refills that gap and the pedal is soft until pumped.

Question 6 of 8 · Hydraulic, Power Assist, and Parking Brake Systems Diagnosis and Repair · 43%Not answered

A vehicle has normal power assist with the engine running, but after the engine is shut off the booster loses its vacuum reserve after only one brake application. The technician removes the check valve and blows through it in each direction. Which result shows the check valve is causing this symptom?

Explanation

Incorrect: A blow-through test applies pressure, not vacuum, so a valve that flows only under vacuum is not a meaningful result of it.

Correct: Air flowing through in both directions means the check valve cannot trap vacuum in the booster, so the reserve leaks back toward the engine as soon as it stops.

Incorrect: A valve that passed air only toward the booster would stop the engine from evacuating it, so assist would be missing even with the engine running.

Incorrect: Flow only from the booster side toward the engine is the normal one-way result of a good valve.

Incorrect: No air flow in either direction means a blocked valve, which would starve the booster and leave no assist with the engine running; that is not the symptom here.

Question 7 of 8 · Hydraulic, Power Assist, and Parking Brake Systems Diagnosis and Repair · 43%Not answered

A vehicle pulls to the right during braking. Tires, alignment, pads, rotors and calipers all check good. Which test best shows whether a hydraulic restriction is the cause?

Explanation

Incorrect: A fluid moisture test concerns the boiling point of the whole system, not side-to-side pressure balance.

Incorrect: A booster drop test checks assist for the whole system, which acts on both sides equally and cannot cause a pull.

Correct: Gauges at both front calipers show whether each side gets the same pressure; a lower left reading points to a restricted hose or line on that side.

Incorrect: Pedal free play is set at the pushrod and affects both sides equally.

Question 8 of 8 · Disc Brake Diagnosis and Repair · 24%Not answered

With the hub flange cleaned and the lug nuts torqued to specification, a technician measures rotor lateral runout on the vehicle at 0.008 in., far above the manufacturer's limit. The same rotor runs true when checked on a bench lathe. What is the most likely cause of the high on-car reading?

Explanation

Incorrect: A sticking piston seal affects pad retraction and drag, not how the rotor turns on its hub.

Incorrect: Glazed pads change friction and noise; they have no effect on a dial indicator reading taken on the rotor face.

Incorrect: Caliper mounting bolts hold the caliper, not the rotor, so they do not move the rotor face under the dial indicator.

Incorrect: Thickness variation is a micrometer measurement of the rotor itself; it cannot make a rotor that runs true off the car wobble on it.

Correct: Bearing play lets the hub and rotor wobble on the car, so the dial indicator reads the bearing plus the rotor; a rotor that runs true off the car points to the hub and bearing side.

Answer every question to see your score

Two habits for every symptom

  1. Isolate when it happens. Only when braking, or also when coasting? Only on a hard stop, or on every stop? Only with the engine off? Each answer removes a system.
  2. Separate hydraulic from mechanical. The task list splits drum and disc diagnosis the same way: one task for hydraulic causes, one for mechanical ones. Ask which half fits the complaint, and test that split before you replace parts.

Written for technicians

This page is study material for the A5 test. If your own car pulses, pulls or sinks when you brake, have it inspected before you drive it further.

The four content areas

Brake symptoms, quick answers

Which of the following can cause brake pedal pulsation?

On a test item: excessive rotor runout or thickness variation, an out-of-round drum, or a loose wheel bearing. Tire pressure, sticking slides and a seized piston show up as pull or uneven pad wear instead.

Why do my brakes pulsate when I stop?

On a normal stop, a rotor of uneven thickness is the first thing to check, then runout and the wheel bearing. On a hard or slippery stop, the pulse can be the ABS working as designed.

What are the signs of a failing caliper?

A pull to one side under braking, a dragging wheel that runs hot, uneven wear between the two pads of one caliper, and fluid at the piston seal.

Why do my brakes grab at low speeds?

Contaminated linings, a loose caliper or a faulty valve in the ABS hydraulic unit can each make one wheel grab under light pedal. On drum brakes, check for a leaking wheel cylinder.