A brake drum is basically a shallow cast-iron cylinder. The inside drum of the cylinder is a machined surface that the brake shoes press against when applied. Most of the reasons to replace a brake drum have to to with the condition of that drum surface; each time the brakes are applied they create friction and heat and wear the drum a little bit. Over time a brake drum can become out of round, or it can wear to an excessively large diameter. Rear brake shoes and brake drums wear very slowly, as compared to disc brakes, but if a brake shoe wears down to the metal, or if an emergency brake is left applied, a brake drum can be easily damaged.
Usually during a brake job the drum is removed and machined to provide a smooth inner surface for the new brakes to work against. They are also measured across the inside diameter to make sure they are still usable; there is a specification for the maximum inside diameter, and if a drum can’t be machined within that spec it needs to be replaced.
Drum brakes used to be the main type of brake system used, both front and rear. Disc brakes have the advantage of being more responsive as well as running cooler, so almost all vehicles since the early 70’s use disc brakes in the front, which does the most work. When ABS brakes became the norm drum brakes again declined in use, as they don’t respond as rapidly and don’t work as well in an ABS system.
But there are still many light trucks and economy cars that use rear drum brakes.
Cost of Brake Drum Replacement
In the great majority of cases, replacing a brake drum will occur in conjunction with a brake job. Then the cost will be for the part only, as no extra labor is involved. A brake drum on a passenger car costs, on average, about $35. A brake drum for a light truck costs, on average, about $70.
If a brake drum is replaced separately from a brake job, it will usually be done in conjunction with a brake adjustment, and be charged out at about an hour of labor. The average cost to replace brake drums on a passenger vehicle would then be about $170, and for a light truck it would be about $240.
For some specific examples on common vehicles, using $100 an hour as a labor rate:
For a 2008 Toyota Tacoma, to replace the rear brake drums and adjust the brakes takes about an hour of labor. A factory brake drum costs about $127, and a Wagner premium drum costs about $60. That makes the job about $354 using OE parts, or about $220 using aftermarket parts.
For a 2006 Kia Spectra, the labor to replace rear drums and adjust he brakes is about an hour. A factory drum costs about $75, and a Raybestos drum costs about $25. That makes the job to replace the rear drums about $250 using OE parts, or about $150 using aftermarket parts.
For a 2008 Chevrolet Cobalt, to replace the rear drums and adjust the brakes takes about an hour of labor. A factory rear drum costs about $157, or a Bendix drum costs about $28. That makes the job about $414 using OE parts, or about $156 using aftermarket parts.
How to Tell if You Might Need Brake Drums
The main indicators would have to do with how the brakes work. If there is squeaking or grinding, or if the brakes grab too hard or don’t grab hard enough, then the brakes would be inspected and measured to see if new rear drums were called for. Other symptoms might be shaking when the brake is applied, indicating the drums could be out of round. If the emergency brake doesn’t hold well that can indicate the brakes are out of adjustment, or that they are worn out, or that the drums are worn past specification. Most of the time an ordinary brake inspection would determine with the problem.
No, rust on the outside of the drum is unavoidable, unless it’s specially coated (which is rare); rust on the outside doesn’t hurt anything. Drum brakes are usually shielded to keep moisture out of the inside except in cases of submersion. Even then, usually, the brakes dry out ok and no harm is done.
They have to be measured across their inside diameter. The specs for maximum size are usually stamped into the drum, and they should be measured with an inside caliper that goes down to thousandths of an inch. Machine shops and many parts stores would have a measuring tool.
Almost the life of a vehicle, unless damaged by a failed component or having brake shoes run down to the metal. They wear very slowly as compared to brake rotors.