Drum and Disc Brake Service Flashcards

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

A rotor measures 0.980 in. thick. The discard thickness marked on the rotor is 1.000 in. What should the technician do?

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.

Before measuring lateral runout of a front disc brake rotor with a dial indicator, what should the technician do first?

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.