Drum Brakes Explained: Shoes, Drums, Springs, Adjusters, and Parking Brake Links

1) What You’re Looking At: The Drum Brake Layout

A drum brake is a “closed” brake: the friction parts live inside a metal drum that rotates with the wheel. When you press the brake pedal, hydraulic pressure pushes the shoes outward so their linings rub the inside of the drum.

Core parts and where they sit

• Backing plate: The fixed metal plate bolted to the axle/spindle. Everything mounts to it.
• Brake drum: The round “bowl” that spins with the wheel. The inside surface is the friction surface.
• Wheel cylinder: Mounted near the top of the backing plate. It has two pistons that push the shoes outward when brake fluid pressure arrives.
• Primary shoe (front shoe): Usually the shoe toward the front of the vehicle. Often has a shorter lining. It reacts first as the drum rotates forward.
• Secondary shoe (rear shoe): Usually toward the rear. Often has a longer lining and does more work in forward braking.
• Return springs: Strong springs that pull the shoes back off the drum when you release the pedal.
• Hold-down pins/springs: Hardware that holds each shoe against the backing plate so it doesn’t flop around, while still allowing it to slide slightly.
• Adjuster mechanism (star wheel): A threaded screw assembly at the bottom between the shoes. Turning it lengthens or shortens the assembly to set shoe-to-drum clearance.
• Self-adjuster lever/cable (common design): A lever that can rotate the star wheel during certain brake applications to maintain clearance as linings wear.
• Parking brake lever/link: A mechanical lever attached to one shoe (often the secondary) that spreads the shoes when the parking brake cable is pulled.

Simple mental map (top-to-bottom)

Top: wheel cylinder pushing both shoes outward. Middle: shoes ride on raised pads of the backing plate and are held by hold-down hardware. Bottom: adjuster assembly spans between shoes; return springs connect shoes to retract them.

2) Self-Energizing (Servo) Action: Why Drums Can Feel “Grabby”

Drum brakes have a characteristic called self-energizing (also called servo action). In forward motion, the rotating drum tends to “pull” the leading shoe into the drum once contact begins. That extra pull increases braking force without needing as much hydraulic push.

How it works in plain terms

• As the drum rotates forward, it drags the primary (front) shoe in the direction of rotation.
• That dragging force wedges the shoe harder into the drum surface.
• Through the shoe-to-shoe link at the bottom (often via the adjuster and anchor points), some of that force transfers so the secondary shoe also presses harder.

This is why rear drum brakes can provide strong braking with relatively small wheel cylinders, and why they can feel more sensitive to adjustment than disc brakes.

What self-energizing changes for you

• Pedal feel and response: If shoes are adjusted close to the drum, the brake can feel responsive and sometimes “grabby” at low speeds.
• Adjustment sensitivity: Too much clearance means the shoes must travel farther before contact, causing long pedal travel. Too little clearance can cause constant light contact and heat.
• Uneven behavior side-to-side: If one side is adjusted tighter or has contamination, the self-energizing effect can amplify the difference.

Why adjustment matters more as shoes wear

As lining wears, the shoe sits farther from the drum. The wheel cylinder must move more fluid to push the shoes out that extra distance. That extra movement shows up as more pedal travel. Many drum systems use a self-adjuster to reduce how often manual adjustment is needed, but the mechanism can stick or be assembled incorrectly.

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3) Parking Brake Integration and Typical Failure Points

Most rear drum brakes double as the parking brake. Instead of hydraulic pressure, the parking brake uses a cable that pulls a lever inside the drum to spread the shoes.

What happens when you pull the parking brake

• The parking brake cable pulls a lever attached to one shoe (commonly the secondary shoe).
• The lever pushes the shoe outward and, through links/struts, forces the other shoe outward too.
• The shoes press against the inside of the drum and hold the wheel from turning.

Common parking brake failure points

• Stretched or seized cable: A stretched cable gives weak hold; a seized cable can prevent full release and cause dragging.
• Frozen lever pivot: Rusted lever pivot points reduce force or keep the brake partially applied.
• Misrouted or missing strut/link: If the internal link/strut is installed wrong, the parking brake may move but not spread both shoes effectively.
• Worn shoes or out-of-adjustment drums: Even with a perfect cable, excessive shoe-to-drum clearance makes the parking brake weak.
• Contaminated linings: Brake fluid or gear oil on the shoes dramatically reduces parking brake holding power.

4) Signs of Misadjustment (and What They Usually Mean)

Long pedal travel

Typical meaning: Shoes are too far from the drum, self-adjuster isn’t working, or drums/shoes are worn. The wheel cylinder must move farther before the shoes touch the drum.

Quick reality check: If the parking brake also feels weak (needs many clicks or won’t hold), that often points to excessive clearance at the shoes.

Weak parking brake

Typical meaning: Out-of-adjustment shoes, stretched cable, seized cable housing, contaminated shoes, or incorrect hardware assembly.

Note: A parking brake that “moves normally” at the lever/pedal can still be weak if the shoes are far from the drum.

Dragging, overheating, or a burning smell

Typical meaning: Shoes adjusted too tight, return springs weak/broken, parking brake cable not releasing, or seized wheel cylinder pistons holding pressure at the shoes.

What you might notice: One rear wheel hotter than the other after a short drive, reduced fuel economy, or the vehicle feels sluggish.

Pulling to one side during braking

Typical meaning: One drum is doing more work due to tighter adjustment, contamination differences, a leaking wheel cylinder, or hardware issues that prevent one shoe from applying correctly.

5) Safe Opening and Inspection: Step-by-Step (Including Dust Precautions)

Drum brakes contain fine dust from normal wear. Treat all brake dust as harmful. Work slowly and avoid creating airborne dust.

Before you start: safety and setup

• Park on a level surface, transmission in Park (or in gear for manual), and chock the front wheels.
• Release the parking brake fully (important—otherwise the drum may not come off).
• Loosen lug nuts slightly before lifting.
• Lift the vehicle at approved points and support it with jack stands. Never rely on a jack alone.
• Wear safety glasses and a suitable dust mask/respirator.

Removing the drum without forcing it

1. Remove the wheel.
2. Inspect the drum face: Some drums have retaining screws or clips—remove them if present.
3. Try to pull the drum straight off. If it resists, don’t pry aggressively against the backing plate.
4. If the drum is stuck on a wear ridge: Use the adjuster access slot (often on the backing plate) to back off the star wheel. You may need to remove a rubber plug to access it.
5. Tap, don’t smash: Light taps around the drum with a rubber mallet can help break rust bonds.

Tip: If the drum only comes off with heavy force, that’s a clue the shoes may be over-adjusted, the parking brake may not be releasing, or there’s a deep ridge inside the drum.

Dust precautions while opening

• Do not use compressed air to blow out the assembly.
• Use brake cleaner to wet the dust and let it drip into a catch pan, or use a damp disposable towel to wipe surfaces carefully.
• Keep your face out of the “cloud zone” as the drum comes off.

What to inspect first (fast triage)

• Overall dryness: The assembly should be dry. Any wetness needs explanation.
• Shoe lining thickness: Compare left vs right. Uneven wear suggests hardware/adjustment issues.
• Hardware condition: Springs should look intact, not stretched, overheated (blue/purple tint), or rubbing abnormally.
• Drum condition: Look for heavy scoring, heat spots, or a pronounced ridge at the open edge.

How to recognize a leaking wheel cylinder

A wheel cylinder leak usually shows as brake fluid seeping from the rubber boots at the ends of the cylinder or running down the backing plate.

• Visual signs: Wet, shiny areas near the wheel cylinder; fluid trails; damp dust stuck to surfaces forming dark paste.
• Shoe signs: The top portion of one or both shoes may look darker, swollen, or glazed where fluid soaked in.
• Smell/feel: Brake fluid feels slick and doesn’t evaporate like water.

Practical check: Gently peel back the wheel cylinder dust boot. If fluid drips or the area is wet inside, the cylinder is leaking.

How to recognize contaminated linings (and why it matters)

Contamination reduces friction and can cause grabbing, noise, pulling, and weak parking brake.

• Brake fluid contamination: Often near the top (wheel cylinder area). Lining may look swollen or patchy-dark.
• Gear oil contamination (common on some rear axles): Often slings outward and coats the lower area and inside of the drum. Strong sulfur/gear-oil smell, very oily feel.
• Grease contamination: Can come from improper handling or nearby service work; usually localized smears.

Inspecting the adjuster and self-adjuster mechanism

1. Locate the star wheel adjuster at the bottom between the shoes.
2. Check orientation: Many adjusters are side-specific (left vs right). If swapped, the self-adjuster may loosen instead of tighten.
3. Check movement: The star wheel should turn with firm, not seized, resistance. If frozen, it won’t maintain clearance.
4. Check the lever/cable: The lever should sit against the star wheel teeth and move when the mechanism is actuated (design varies). A missing spring or wrong routing can prevent adjustment.

Inspecting springs and hold-down hardware

• Return springs: Look for distortion, corrosion, or signs of overheating. Weak springs can allow dragging.
• Hold-down pins/springs: Shoes should sit flat against the backing plate pads. Broken hold-downs can let a shoe tilt and wear oddly.
• Backing plate contact pads: Where the shoes rub the backing plate, look for deep grooves. Grooves can cause sticking and uneven apply/release.

Parking brake linkage checks inside the drum

1. Locate the parking brake lever attached to the shoe and the link/strut that transfers force.
2. Move it by hand (with the cable slack). It should pivot smoothly and return when released.
3. Check for missing clips or incorrect assembly: A small missing retaining clip can let the lever bind or misalign.
4. Check the cable end: It should seat properly and not be frayed.

Reinstalling the drum for a basic adjustment check (no special tools)

If you are only inspecting and reassembling, you can do a basic clearance check:

1. Clean the drum friction surface with brake cleaner and wipe dry.
2. Install the drum and rotate it by hand.
3. Feel for light, even drag (varies by design). No contact at all may indicate under-adjustment; heavy drag indicates over-adjustment or a release problem.
4. If adjustment is needed, use the access slot to turn the star wheel in small steps, rechecking rotation frequently.