Drum Brake Systems Brake Shoe Energization When the

Drum Brake Systems

Brake Shoe Energization • When the brake shoes are forced against the rotating drum, they are pulled away from their pivot point by friction • This self-energizing action draws the shoes tighter against the drum • Servo action results when the primary shoe helps apply the secondary shoe

Servo • Anchor at top acts as a pivot • Shoes linked by adjuster • Primary shoe forced against secondary shoe

Non-Servo • Anchor prevents energization • Most of wear is in top half of shoes • Typically smaller vehicles

Which style increases braking force? 1. Servo 2. Non-servo

Type of brake? 1. Servo 2. Non-Servo

Backing Plate • Holds the springs, wheel cylinder, and other parts inside the drum • Helps keep road dirt and water off the brakes • Bolts to the axle housing or the spindle support

Drum Brake Components • Backing plate – Holds the drum components

Backing Plate

Wheel Cylinder Assembly Uses master cylinder pressure to force the brake shoes out against the drum

Wheel Cylinder Components • Boots – keep road dirt and water out of the cylinder • Pistons – metal or plastic plungers that transfer force out of the cylinder to the brake shoes • Cups – rubber seals that keep fluid from leaking past the pistons

Wheel Cylinder Components • Springs – hold the rubber cups against the pistons when the wheel cylinder assembly is not pressurized – metal expanders may be used on the ends • Bleeder screw – provides a means of removing air from the brake system

Purpose of the spring? 1. 2. 3. 4. 5. 6. 7. 8. Retract pistons Retract shoes Retract cups Keep cups out against pistons Keep pistons from retracting too far Maintain correct pedal travel 1– 3 4– 6

Wheel Cylinder Inspection

Brake Shoes • Rub against the revolving brake drum to produce braking action • Made by fastening organic friction material onto a metal shoe – rivets or bonding agents may be used • The front shoe is the primary shoe – uses the shortest lining • The rear shoe is the secondary shoe – uses the largest lining surface area

All brake shoes are glued on? 1. Yes 2. No

Brake Springs • Retracting springs – pull the brake shoes away from the brake drums when the brake pedal is released • Hold-down springs – hold the brake shoes against the backing plate when the brakes are in the released position

Different size brakes will have different size springs? 1. True 2. False

Brake Shoe Adjusters • Maintain the correct drum-to-lining clearance as the brake linings wear • Automatic adjusters normally function when the brakes are applied with the vehicle moving in reverse • If there is too much lining clearance, the brake shoes move outward and rotate with the drum enough to operate the adjuster lever

Adjuster Screw Parts

Brake Shoe Adjusters Cable-type star wheel adjuster

Brake Shoe Adjusters Lever-type star wheel adjuster

Service Precautions • Allow the brakes to cool before servicing them • Familiarize yourself with the brake system before attempting service • Follow the manufacturer’s service procedures

Service Precautions • Do not reuse damaged parts • Do not reuse drums that are damaged or beyond service limits

Drum Brake Service Tools • Vacuum enclosure or wet sink – Used to clean brake dust from brake components

Clean Dust – DO NOT BLOW IT OFF!!!!!!

Approved Method

Remove As Much As Possible

Vacuum Enclosure

Tools • • Spoon – adjusting Clamp – holds pistons Hold down tool Return spring tools

Lining Minimum Thickness

Lining Thickness Measurement

Lining Thickness Gauges

Adjustment

Drum Brake Service Tools • Drum brake micrometer – Used to measure drum diameter and check for out of round

Drum Measurement 1. Determine the maximum drum diameter and the machine-to limit

Drum Measurement 2. Set the bar of the micrometer to the nominal diameter 3. Place the micrometer into the drum and move until the highest reading on the dial is obtained

Drum Measurement 4. Add the gauge reading to the reading on the bar 5. Repeat the procedure at ninety degree points around the drum

Brake Drum Measurement Click on image above to view animation

Drum Defects

Drum Wear Patterns

Drum Brake Concerns • Noise – Can be caused by: • Brake dust accumulation • Worn linings • Dry backing plate studs • Broken parts

Drum Brake Concerns • Grabbing – Can be caused by: • Overly tight shoes • Fluid contamination on the linings

Drum Brake Concerns • Parking brake does not hold – Can be caused by: • Improperly adjusted shoes • Stuck or binding lever or cables

Drum Brake Concerns • Pulsation – Commonly caused by an out of round brake drum

Drum Brake Concerns • Low, soft, or spongy brake pedal – Usually caused by a leak in the hydraulic system

Wheel Cylinder Inspection

Leaking Wheel Cylinder or Axle Seal?

Drum in Hat

Drum in Hat