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Drum brakes turn outward shoe force into braking torque
A wheel cylinder or mechanical lever moves the shoes until their linings contact the rotating drum. Friction creates tangential force and the backing plate or anchor reacts the resulting torque. The enclosed design provides a large friction area and can integrate a durable parking brake.
Drum geometry also creates self-energising action: rotation can pull a leading shoe more firmly into contact. That improves output for a given hydraulic force but makes shoe orientation, adjustment and friction matching critical.
Operating sequence
- Pedal force creates hydraulic pressure in the brake circuit.
- Wheel-cylinder pistons push the upper or designated shoe ends apart.
- Shoes pivot or slide at their lower anchor/adjuster.
- Linings contact the rotating drum and produce friction torque.
- Self-energising geometry increases force on the leading shoe where designed.
- When pressure falls, return springs pull the shoes away.
- The adjuster preserves a small clearance as the linings wear.
Drum brake layouts
| Layout | Arrangement | Service characteristic |
|---|---|---|
| Leading/trailing | One shoe leads and one trails in forward rotation. | Shoe positions and parking lever must match design. |
| Twin leading shoe | Two cylinders or mechanisms make both shoes leading forward. | Strong forward braking; reverse behaviour differs. |
| Duo-servo | Shoes link through a floating adjuster and react together. | Primary/secondary lining orientation is critical. |
| Simplex/anchor-based | Shoes pivot from a fixed anchor opposite the cylinder. | Anchor and spring seating control movement. |
| Drum-in-hat parking brake | Small mechanical shoes act inside a rear disc hat. | Service brake remains disc-operated. |
| Electric parking-brake drum mechanism | Motor/cable actuator expands dedicated shoes. | Service mode and actuator adjustment may be required. |
Components around the shoes
Backing plate
The plate locates wheel cylinder, anchor, adjuster and shoe support pads. Deep grooves at contact pads make shoes catch rather than slide smoothly. Severe corrosion, cracks or distorted mounting points require replacement.
Wheel cylinder
Hydraulic pressure moves two pistons beneath rubber boots. Dampness behind a boot, seized pistons or bore corrosion can cause leakage and imbalance. A leaking cylinder contaminates linings and is not repaired by replacing shoes alone.
Return and hold-down springs
Return springs establish release while pins, cups and small springs hold shoe webs against the backing plate. Heat and corrosion weaken them. Correct colour and shape identification applies only to the specific kit diagram.
Adjuster
A threaded star wheel or ratchet lengthens as linings wear. Handed threads, lever orientation and assembly length differ left to right. A seized or backwards adjuster defeats automatic compensation.
Parking-brake lever and cable
The cable moves a lever attached to one shoe or a strut between shoes. Lever clips and cable ends must be secure, and the cable must return freely without holding the shoe on.
Shoe construction and friction materials
A pressed steel web and table support the curved lining. The lining is bonded or riveted and may have chamfers, slots and different-length segments to control engagement. Do not grind a new lining to force it into an unsuitable drum.
Friction formulations balance cold response, fade, wear, noise and drum compatibility. Axle sets must use matched shoes. Mixing old and new formulations side to side can create imbalance even where dimensions appear identical.
Contamination by brake fluid, axle oil or grease changes friction unpredictably and penetrates porous lining. Replace contaminated shoes as an axle set and repair the leak; surface cleaning is not a reliable restoration.
Fitment evidence
| Check | Possible variation | Why it matters |
|---|---|---|
| Axle/brake code | Different drum systems on the same model. | Defines shoe and hardware geometry. |
| Drum internal diameter | Standard or heavy-duty package. | Sets shoe curvature and adjustment range. |
| Shoe width | Load and braking-capacity variation. | Must fit backing plate and drum track. |
| Lining position | Primary/secondary or equal-length shoes. | Self-energising action depends on orientation. |
| Adjuster design | Manual, automatic and left/right thread. | Controls clearance and self-adjustment. |
| Parking lever | Pre-fitted, transferable or separate. | Must pivot and retain correctly. |
| Pack coverage | One wheel, one axle or parking set. | Determines quantity and matched replacement. |
| Production date | Wheel cylinder, spring or anchor revision. | Prevents mixing incompatible hardware. |
Drum condition and operating limits
Measure internal diameter at several positions with suitable equipment. The drum's maximum diameter is a wear and machining limit, not a target size. A bell-mouthed, oval, heat-spotted or cracked drum will not provide uniform contact.
A wear ridge can trap expanded shoes and complicate removal. Back off the adjuster through the intended access rather than forcing the drum over hardware. If machining is permitted, both drums must remain within limits and have a suitable finish; replacement is often preferable.
Remove preservative coating from new friction surfaces with approved brake cleaner before assembly. Keep fingerprints, grease and grinding debris away. Drum balance weights must remain attached.
Adjustment and pedal travel
Excessive shoe-to-drum clearance consumes wheel-cylinder travel before braking begins, increasing pedal movement and weakening parking-brake action. Too little clearance causes drag, heat and rapid wear. Automatic adjusters still need correct initial setup and functioning cable or lever movement.
Adjust shoes first, then the parking-brake cable where the procedure requires it. Tightening the cable to compensate for unadjusted shoes prevents release and leaves the self-adjuster outside its intended range. Some systems set clearance by repeated pedal or parking-brake operation after initial manual adjustment.
Diagnosis by symptom
| Symptom | Possible drum-brake cause | Other checks |
|---|---|---|
| Pulling under braking | Contamination, cylinder seizure, adjustment or friction difference. | Front brakes, tyres and hydraulic restrictions. |
| Long pedal travel | Excess shoe clearance or hydraulic air/leak. | Entire brake circuit and master cylinder. |
| Poor parking-brake hold | Worn shoes, poor adjustment, cable or drum condition. | Actuator and lever travel. |
| One drum overheats | Seized cylinder/adjuster, cable or weak return spring. | Hose restriction and wheel bearing. |
| Grab when first applied | Contamination, rust, incorrect shoe or rough drum. | Moisture history and suspension movement. |
| Scrape or knock | Broken spring, loose lining or displaced hardware. | Stop use and inspect immediately. |
| Fluid inside drum | Wheel-cylinder leak. | Identify brake fluid versus axle oil. |
| Axle oil contamination | Hub or axle seal failure. | Bearing play and axle vent. |
Dust and hazardous-material control
Never use compressed air or dry brushing on brake dust. Older, imported or unidentified friction material may contain asbestos, and modern dust is still harmful. Use an approved low-pressure wet cleaning or specialist brake-cleaning system and suitable personal protective equipment.
Do not sand linings in open air. Bag removed parts and contaminated wipes according to workplace and waste rules. Wash hands before eating and avoid carrying dust into vehicles or homes on clothing.
Inspection sequence
- Road-test only if braking remains safe and record pull, noise and pedal feel.
- Secure the vehicle and release the parking brake as the procedure requires.
- Back off the adjuster where a drum ridge prevents removal.
- Control dust before touching the assembly.
- Photograph spring, adjuster and lever positions on each side.
- Measure lining, drum, wheel-cylinder and backing-plate condition.
- Inspect hydraulic hoses, pipes, hub seal and bearing.
- Compare both sides for unequal wear and incorrect prior assembly.
Installation procedure
Work on one side at a time so the other remains a reference, but recognise that previous assembly may be wrong. Use the current brake diagram. Keep primary and secondary shoes, handed adjusters and parking levers identified.
Clean backing-plate contact pads and apply only the specified trace of compatible brake lubricant to metal sliding points. No compound belongs on lining or drum. Renew springs and retainers where required, and ensure each spring sits in its intended hole without rubbing a hub or cylinder boot.
Fit the adjuster in the correct direction and initial length. Install the drum without forcing it, set clearance, operate the brake to centre shoes and recheck rotation. Bleed only if the hydraulic circuit was opened, using the specified sequence and fluid.
Bedding and post-service checks
Before moving, pump the pedal until firm, check fluid level and apply/release the parking brake. Confirm both wheels rotate with only the specified light contact and that cables return. Tighten wheel fasteners in sequence.
Bed new shoes using the manufacturer procedure, avoiding prolonged parking-brake application while drums are very hot. Friction surfaces need controlled contact to conform; severe early braking can glaze linings and create hot spots. Recheck adjustment where specified.
A brake tester or controlled road test should confirm axle balance, parking-brake performance and absence of overheating. Stop immediately for pull, smoke, a soft pedal or abnormal noise.
Common mistakes
- Blowing brake dust out with compressed air.
- Replacing shoes on only one side of an axle.
- Reversing primary and secondary shoes.
- Mixing left- and right-hand adjusters.
- Reusing heat-weakened or corroded springs.
- Cleaning and reusing fluid- or oil-contaminated linings.
- Lubricating friction surfaces or using excessive compound.
- Tightening the parking cable before adjusting shoes.
- Ignoring drum maximum diameter and ovality.
- Moving the vehicle before pumping the pedal firm.
UK MOT and brake safety
Service and parking brakes are tested for operation, efficiency and imbalance according to vehicle class and current MOT criteria. Worn, contaminated, insecure or incorrectly assembled shoes can contribute to failure even if the pedal initially feels normal.
Do not drive with a fluid leak, grinding, severe pull, overheating, loose lining or failed parking brake. Brake work should be performed by a competent person using suitable supports, torque data and test equipment. A pass at one moment does not replace regular inspection of concealed drum components.
Brake shoe FAQs
Q: What do brake shoes do?
A: Their curved linings press against a drum to create braking torque.
Q: Are drum shoes and parking-brake shoes the same?
A: Not always; disc brakes can use separate small drum-in-hat parking shoes.
Q: Should shoes be replaced in axle sets?
A: Yes, to maintain matched friction and braking balance.
Q: Can contaminated shoes be cleaned?
A: No reliable cleaning restores absorbed brake fluid, oil or grease.
Q: Must spring hardware be replaced?
A: Follow service guidance and renew weakened, corroded or specified parts.
Q: Why are primary and secondary shoes different?
A: Their lining layout controls self-energising action in applicable designs.
Q: Can compressed air clean drum brakes?
A: No. Use approved dust-control methods.
Q: Why is the brake pedal travel long?
A: Excess clearance, hydraulic air, leaks or other circuit faults are possible.
Q: Why does one drum become hot?
A: Check cylinder, springs, adjuster, cable and hydraulic release.
Q: Should the cable be tightened to improve adjustment?
A: Adjust the shoes first and follow the specified sequence.
Q: Can a worn drum be reused with new shoes?
A: Only if dimensions, surface and shape remain within limits.
Q: Do new shoes need bedding?
A: Yes, using the controlled procedure specified for the vehicle.
Q: Can worn brake shoes fail the MOT?
A: Yes through low efficiency, imbalance, insecurity or associated defects.