ASE A5 Drum and Disc Brake Service — Questions and Answers
Question 1: A vehicle with rear drum brakes has a low brake pedal on the first application. The pedal rises and feels firm when it is pumped, and the fluid level and front brakes are normal. Which is the most likely cause?
- Air trapped in the master cylinder reservoir
- A restricted rear brake hose
- An inoperative drum brake self-adjuster (Correct answer)
- A leaking wheel cylinder dust boot only
Correct answer: An inoperative drum brake self-adjuster
Excessive shoe-to-drum clearance means the pedal must travel farther to take up the gap. Pumping the pedal closes that gap and brings the pedal up. A self-adjuster that doesn't work leaves the shoes too far from the drum.
Drum brakes rely on a self-adjuster, usually a star wheel with a lever or cable, to keep shoe-to-drum clearance small as the lining wears. When the adjuster is seized, misassembled, or not actuated, clearance grows. The first pedal stroke then spends most of its travel moving the shoes outward, so the pedal feels low. A second stroke starts with the shoes already close to the drum, so the pedal comes up higher and firmer. A hydraulic fault such as air or a leak usually gives a soft or sinking pedal that pumping doesn't fix as consistently. After repair, set the initial star-wheel adjustment and confirm the adjuster operates.
Question 2: A technician measures a brake drum and finds the friction surface is larger than the maximum diameter cast into the drum. What is the correct action?
- Replace the drum (Correct answer)
- Install thicker linings to compensate
- Machine the drum lightly to true the surface
- Reuse the drum if it has no hard spots
Correct answer: Replace the drum
The cast discard diameter is the largest safe inside diameter. A drum beyond it has less metal to absorb heat and resist cracking, so it must be replaced.
The maximum diameter marked on a drum is a discard dimension, not a target for machining. It is the limit at which the drum still has enough mass to absorb braking heat and enough strength to resist the outward force of the shoes. A drum worn or turned past this limit runs hotter, is more likely to fade, and can crack or distort. Thicker linings or lighter machining cannot restore the lost metal. Measure the drum with a drum micrometer or vernier gauge, compare the reading to the cast specification, and replace the drum if it is over.
Question 3: In a duo-servo drum brake with a primary and a secondary shoe, why is the secondary shoe lining typically longer or thicker than the primary shoe lining?
- The primary shoe does all of the parking brake work
- The secondary shoe is the only shoe the wheel cylinder pushes
- The secondary shoe is cooler, so it needs less friction material
- The secondary shoe does most of the braking, so it wears more (Correct answer)
Correct answer: The secondary shoe does most of the braking, so it wears more
In a duo-servo design, the primary shoe drives the secondary shoe into the drum through the adjuster. The secondary shoe therefore handles most of the braking force and wears more.
In a duo-servo brake, the wheel cylinder pushes both shoes out. The primary shoe, which faces the front of the vehicle, is dragged by drum rotation and pushes on the secondary shoe through the star-wheel adjuster. This self-energizing action multiplies the force on the secondary shoe, which then does the larger share of the work and wears faster. For this reason the secondary lining is made longer or thicker. Installing the shoes in the wrong positions can cause uneven wear, noise, and poor braking.
Question 4: After a brake job, a customer reports squeaking from a rear drum brake during light braking. The linings and drum are new. Which service step was most likely missed?
- Machining the drum to a mirror finish
- Applying high-temperature brake lubricant to the backing plate shoe contact points (Correct answer)
- Coating the lining surface with brake grease
- Tightening the star-wheel adjuster until the drum drags
Correct answer: Applying high-temperature brake lubricant to the backing plate shoe contact points
The shoe edges slide on raised pads on the backing plate. When those contact points are dry or rusty, the shoes stick and chatter, which causes noise. A thin film of high-temperature brake lubricant fixes this.
The backing plate has raised ledges or pads that support the shoe webs and let them slide in and out as the brake applies and releases. If these areas are rusty or dry, the shoes bind and release unevenly, which can cause squeaks, uneven wear, and slow return. During a drum brake job, clean the ledges and apply a small amount of high-temperature brake lubricant. Keep the lubricant off the linings and drum friction surface. Check the hold-down hardware and adjuster threads at the same time.
Question 5: A vehicle has a steering wheel and brake pedal pulsation that appears only during braking and gets worse at highway speeds. The pads and fluid are normal. Which condition is the most likely cause?
- Weak brake booster vacuum supply
- A restricted rear brake hose
- Air in the hydraulic system
- Disc thickness variation from rotor runout (Correct answer)
Correct answer: Disc thickness variation from rotor runout
Rotor thickness variation makes the pads push back and forth as the rotor turns. This shows up as a pulsing pedal and, on front brakes, steering wheel shake.
Lateral runout and uneven pad deposits can wear the rotor into spots of different thickness. As each thick spot passes between the pads, the caliper piston is pushed back slightly and the pedal pulses. Front rotors also transmit the pulsing to the steering wheel. Measure runout with a dial indicator and thickness at several points with a micrometer. If the rotor is within the minimum thickness, it may be machined or replaced. Check hub condition and wheel torque, because uneven torque can also cause distortion.
Question 6: A vehicle pulls to the right whenever the brakes are applied. The brake pedal feels normal and there is no noise. Which condition is the most likely cause?
- A seized caliper piston on the left front brake (Correct answer)
- Glazed pads on the right front brake
- Brake fluid contamination on the right front pads
- A weak return spring on the right rear shoe
Correct answer: A seized caliper piston on the left front brake
A vehicle pulls toward the side with more braking force. A seized left caliper piston reduces braking on the left, so the right side does more of the work and the car pulls right.
Brake pull happens when the left and right front brakes do not apply equal force. The vehicle steers toward the side with more braking. If the left caliper piston is stuck or the slide pins are seized, the left pads can't clamp the rotor properly, leaving the right side stronger. Glazed or contaminated pads on the right side would weaken that side, so the car would pull left. When diagnosing pull, check caliper piston movement, slide pins, hoses, pad condition, and tire pressure and size side to side.
Question 7: After driving, one front wheel is much hotter than the other and the vehicle pulls to that side when coasting. When the caliper bleeder is opened, the wheel spins freely. What is the most likely cause?
- A warped rotor on the opposite wheel
- Excessive master cylinder reservoir fluid
- A collapsed inner lining in the brake hose (Correct answer)
- A worn pad wear indicator
Correct answer: A collapsed inner lining in the brake hose
A damaged hose can act like a one-way valve. It lets fluid in under pedal pressure but doesn't let it return, so the pads keep clamping the rotor until the bleeder releases the trapped pressure.
The inner liner of a flexible brake hose can deteriorate and flap inward. Pedal pressure can push fluid past it, but the weaker return flow can't. Pressure remains trapped at the caliper, so the pads drag and the brake overheats. Loosening the bleeder relieves the pressure and frees the wheel, which points to a restriction in the hose or line rather than a mechanical caliper problem. Replace the hose, then bleed the system and check for correct caliper release.
Question 8: A customer complains of a high-pitched squeal from the front brakes that occurs while driving and stops when the brake pedal is pressed lightly. What is the most likely cause?
- Cracked brake rotor
- Pad wear indicators contacting the rotors (Correct answer)
- Seized caliper slide pins
- Air in the brake lines
Correct answer: Pad wear indicators contacting the rotors
A wear indicator tab on the pad touches the rotor when the lining is nearly worn out. It makes a squeal that often stops when brake pressure is applied.
Many disc pads have a small metal tab that touches the rotor when the lining has worn down to the minimum. The contact produces a high-pitched squeal as a warning. Pressing the brake can push the pad away from the tab, so the noise often stops. When you inspect the brakes, check pad thickness and replace worn pads in axle sets. Also inspect rotors, hardware, and caliper slides, and replace or resurface the rotors as required.
Question 9: A rotor measures 0.980 in. thick. The discard thickness marked on the rotor is 1.000 in. What should the technician do?
- Resurface the rotor and install new pads
- Install shims behind the pads to make up the difference
- Leave the rotor in place and replace only the pads
- Replace the rotor (Correct answer)
Correct answer: Replace the rotor
The rotor is already below the discard thickness, so it no longer has enough mass to absorb heat or resist cracking. It must be replaced.
Every rotor has a minimum thickness, often marked as discard or MIN THK. It is the limit at which the rotor can still dissipate heat safely and keep its strength. A rotor at 0.980 in. is thinner than the 1.000 in. limit, so even new pads are not acceptable with it. Machining removes more metal and would worsen the problem. Measure with a micrometer at several points, and replace the rotor if any reading is below the specification.
Question 10: Before measuring lateral runout of a front disc brake rotor with a dial indicator, what should the technician do first?
- Remove the brake pads and clean the pad area
- Lubricate the caliper slide pins
- Make sure the wheel bearing has no excess play and the rotor is mounted correctly (Correct answer)
- Bleed the brakes to remove all air
Correct answer: Make sure the wheel bearing has no excess play and the rotor is mounted correctly
Looseness in the wheel bearing or poor rotor mounting can give a false runout reading. The bearing and rotor mounting must be correct before measuring.
Lateral runout is the side-to-side wobble of the rotor friction surface as it rotates. A dial indicator is mounted on a stable point and read as the rotor turns. If the wheel bearing is loose, or the rotor is not seated against the hub or is held by uneven lug nut torque, the reading can be wrong. Check bearing play and hub condition, clean the mounting surfaces, and tighten the rotor or wheel to specification. Then measure runout and compare it to the manufacturer's limit.
ASE A5 Drum and Disc Brake Service
A vehicle with rear drum brakes has a low brake pedal on the first application.
The pedal rises and feels firm when it is pumped, and the fluid level and front brakes are normal.
Which is the most likely cause?
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