Rear Drum Brakes: Still on Plenty of Vehicles, Still Need Service

Many trucks, vans and smaller cars still run rear drums. How they wear, why they get ignored, what adjustment means, and what a drum service involves.

Disc brakes get all the attention, but a large number of vehicles on South Plains roads still have drums on the rear axle: older pickups, work vans, compact cars, and a lot of trailers. Drums are durable and cheap, and they are also the most neglected brake on the vehicle because nobody can see them.

How a drum brake works

Inside a cast iron drum that turns with the wheel, two curved shoes are pushed outward by a hydraulic wheel cylinder until their lining rubs against the inside of the drum. Springs pull the shoes back when the pedal is released. An adjuster mechanism takes up the slack as the lining wears so the shoes stay close to the drum. The parking brake usually works through the same shoes with a cable.

Why they get ignored

You cannot see the lining without pulling the drum. Rear brakes do a smaller share of the stopping than fronts, so they wear slowly and go a long time between services. And a drum brake that is out of adjustment degrades gradually, with the pedal getting a little longer over months, which a driver adapts to without noticing.

What goes wrong

• Worn lining, eventually down to the rivets or the steel shoe, which scores the drum.
• Seized adjusters. The self-adjuster corrodes and stops working, the shoes drift away from the drum, and the pedal gets long and the parking brake gets weak.
• Leaking wheel cylinders. Fluid contaminates the lining, the brake grabs or does nothing, and the fluid level in the reservoir drops.
• Broken or weak springs, which let a shoe drag or hang up.
• Drum wear past the maximum diameter stamped inside it, which is the drum equivalent of a rotor below minimum thickness.
• Dust and grit packed inside from caliche roads, which grinds the lining and makes noise.

What a drum service involves

1. 1Pull the drums, which sometimes means backing off the adjuster or using a puller if they are rusted to the hub.
2. 2Measure the drum inside diameter against the stamped maximum and inspect for scoring, heat checking and out-of-round.
3. 3Inspect lining thickness, wheel cylinder for leaks, springs and hardware for wear.
4. 4Clean everything. Brake dust is not something to blow out with an airline.
5. 5Replace shoes as an axle pair, replace the hardware kit, replace or rebuild a leaking cylinder, and machine or replace drums as the measurement dictates.
6. 6Lubricate the contact points on the backing plate, adjust the shoes properly, and bleed if the cylinders were opened.
7. 7Adjust the parking brake cable so it holds within the specified number of clicks.

Signs the rear drums need attention

• A long or low pedal that firms up if you pump it.
• A parking brake that needs pulling to the last click to hold, or does not hold on a grade.
• A scraping or grinding from the rear, especially on the first few stops of the day.
• Rear wheels that are hotter than they should be after a drive.
• A vehicle that pulls or a rear that feels like it locks early on a wet road.

Frequently asked questions

How often should rear drums be checked?

At every front brake job at minimum, and any time the rear wheels are off for a tire rotation is a chance to at least check the adjustment. On a work truck with a loaded bed, more often.

Can drums be converted to discs?

On many vehicles, yes, with a kit. It is rarely worth it for a normal daily driver. A well-serviced drum brake stops the rear axle perfectly well.

Why is the parking brake weak after new shoes?

Usually because the shoes were not adjusted correctly after fitting, or the cable was never taken up. Both are part of the job. The parking brake nobody uses covers the rest.

Front discs and rear drums are both handled in our bays. Call (806) 458-2709 or bring it to 13209 Frankford Ave, Suite 29 in Lubbock for brake service. Drivers make the trip in from Levelland and across the South Plains for it.