Drum brakes may be "old technology" but they're still used for the rear brakes on most cars and trucks. Up until the early 1970s, drum brakes were used both front and rear on almost all vehicles. But in the 1970s, front drum brakes were replaced with disc brakes. Drum brakes were kept in the rear because braking loads are lighter so disc brakes are not really needed. Drum brakes are also less expensive to manufacture than disc brakes, and provide a simpler parking brake setup.
The drum is part of the drum brake assembly, which also includes a wheel cylinder, a pair of shoes, return springs and hold down springs, a backing plate, some type of self-adjuster mechanism to maintain lining clearances, and a parking brake mechanism.
The drum is usually cast iron but some are aluminum with a cast iron liner. The drum provides the friction surface against which the linings rub when the shoes are pushed outward by hydraulic pressure in the wheel cylinder. As the linings rub against the drum, they generate friction to slow the wheel and bring the vehicle to a halt. This generates a lot of heat, which the drum absorbs and dissipates so the linings don't get too hot and begin to fade. Some drums have cooling fins that increase their cooling capacity. The drum also shields the internal brake components from road splash and dirt. Some drums also have a spring wrapped around the outside to dampen vibrations and noise.
Front drums on older rear-wheel drive cars and trucks are mounted on a spindle and supported by a pair of wheel bearings. The rear drums on rear-wheel drive cars and trucks are mounted on the axle hub, while those on front-wheel drive cars and minivans are either mounted on a spindle or hub. Most of these will have sealed wheel bearings, but some may have bearings that require periodic service.
The drums must be removed to inspect or service the rear brakes. The drums should always be removed even if the front brakes are the only ones being serviced. But the rear drums may not be removed to save time and effort, especially if the vehicle has low miles (under 50,000) and is having its first brake job.
The first step in removing a drum is to remove the wheel. Next, the shoes must be backed off so the linings will clear the inside lip of the drum. This is necessary because as the drums wear, they often develop a ridge that will catch on the shoes. On most drum brakes that have a star wheel self-adjuster, a screwdriver or brake adjuster tool can be inserted through a slot in the backing plate to back off the adjuster.
If the drum is stuck to the axle or shoes and does not want to pull off, use a large SOFT hammer to pound on
the front of the drum. Do NOT beat on the drum with a steel hammer because you might crack or chip the cast iron drum. If the drum refuses to budge, you can use a blunt tool in an air hammer to pound on the face of the drum. The vibration should loosen it up so it will come off.
If that doesn't work, try a drum puller. This is a tool that looks like a large gear puller. It has fingers that grip the sides of the drum and a large screw in the middle that is placed against the axle hub.
CAUTION: Drums will sometimes pop free with considerable force when removed with a puller. Also, the drum may fall to the ground and be damaged or smash your toe, so make sure you have a good grip on it when it comes loose. The best way to remove a drum with a puller is to slowly tighten the screw while tapping against the face of the drum with a soft hammer or air hammer to help vibrate it loose. You can also use a propane torch to apply heat around the center hole in the drum to help it expand away from the hub.
On older vehicles with front drum brakes, the drums are usually easier to remove. All you do is back off the brake shoes from behind with a brake adjuster tool or screwdriver, remove the wheel, then the dust cap on the hub, and remove the hub nut from the spindle. The outer wheel bearing will usually fall out of the drum when it comes off. So the outer wheel bearing should be carefully removed and set aside so it and the inner wheel bearings and be cleaned, inspected and repacked with grease before the drum goes back on the vehicle.
AVOID BRAKE DUST
Drums trap a lot of brake dust which may contain asbestos fibers. Exposure to asbestos dust may increase the risk of lung cancer and other lung disease, so care must be used to avoid inhaling the dust or getting it on clothing. The dust should be removed by cleaning the inside of the drums with an aerosol brake cleaner. Never use pressurized air to blow dust off the drums or other brake parts.
Once the drums are off, they should be measured to determine how much they are worn. Stamped or cast on the face or outside of most drums is a "maximum diameter" or "machine to" specification, which refers to the maximum allowable inside diameter of the drum that is permitted AFTER the drum has been resurfaced. The drum may also have a "discard" specification. If the drum is already worn to discard spec, or cannot be turned without exceeding the maximum diameter limit, discard it.
Measuring the inside diameter of a drum requires a special drum gauge. If you don't have a gauge, most auto parts stores that resurface drums will have such a tool, and they should know how to use it (if they don't take your drums someplace else!). If the drum's inside diameter is at or beyond the maximum specification, or cannot be resurfaced without exceeding the maximum specification, the drum is junk and must not be resurfaced. Most passenger car and light truck drums are designed with enough metal to allow about .060 inch of wear (or resurfacing) before replacement is necessary.
Why replace worn drums? Here's five good reasons:
1. Thin drums are dangerous drums. If not replaced, the drum can wear to the point where it may literally disintegrate. And if the drum fails, so do the brakes!
2. Reduced braking efficiency. The increased inside diameter of a badly worn drum won't match up with a new set of linings unless the linings are arced (and nobody does that anymore). Consequently, the linings only make contact in the middle which reduces their effective braking ability.
3. Increased pedal travel. Wear increases the distance between the shoes and drum. If the drum is worn excessively, the self-adjuster mechanisms may not be able to take up all the slack resulting in a low pedal. There's also the chance that the wheel cylinder may overextend to the point where it leaks or comes apart, causing brake failure.
4. Brake fade. Thin drums can't absorb and dissipate as much heat as thick ones. There's less mass in a worn drum so the brakes may overheat and fade much sooner.
5. Noise. Thin drums can contribute to noise by failing to dampen vibrations.
Drums should also be inspected for the following conditions:
* Minor pitting and scoring are acceptable as long as the grooves are not too deep and can be removed by resurfacing. Grease or oil contamination must be removed before the drum is reused because grease can cause grabbing and uneven braking.
* Cracks of any kind are dangerous because they weaken the metal and increase the risk of breakage. Cracked drums should not be reused.
* Out-of-round or an egg-shaped drum can result from applying the parking brake when the drum is hot. The locked position of the shoes prevents the drum from contracting normally as it cools resulting in permanent distortion. This can cause a pedal pulsation when the brakes are applied. Drum runout should not exceed .005 in. If the run-out cannot be removed by resurfacing, replace the drum.
* A "bellmouth" condition can occur in wide drums if the inner edge of the drum distorts outward. The result is uneven shoe-to-drum contact, uneven lining wear and reduced braking efficiency. To check for this condition, measure the inside diameter of the drum close to the inside and outside edges of the drum. If there's a difference of more than .005 in., the drum needs to be resurfaced or replaced.
* Barrel wear on the drum where the center portion is worn more than the edges. If the distortion can't be removed by resurfacing, it will have to be replaced.
* Hard spots or glazed spots in the drum. These are caused by excessive heat that brings about metallurgical changes in the metal. Hard spots can be identified by raised or discolored patches on the drum friction surface. Hard spots can cause chatter, pedal pulsation and grabbing when the brakes are applied. Resurfacing will shave the tops off any hard spots, but they usually come back once the drum is returned to service and starts to wear again. The only permanent cure for hard spots is a new drum.
* Broken or damaged wheel studs. Replace as needed.
Brake drums need to have a relatively smooth friction surface inside. A rough surface will reduce brake life as well as braking efficiency. So if the drums are rough, they will have to be resurfaced or replaced to restore the friction surface to like-new condition.
Drums should always be resurfaced in pairs. Never do one drum and not the other because differences in diameter can affect side-to-side brake balance. The inside diameters of both drums should be within .010 inch of each other. A greater difference may cause undesirable variations in brake force side-to-side.
New drums come from the factory in a finished condition and are ready to install, so additional resurfacing is not necessary and only shortens the potential service life of the drum.
When a drum is mounted on a lathe for resurfacing, it must be mounted straight and secure to minimize runout. A silencer band should be wrapped around the outside of the drum to dampen vibrations and reduce the possibility of creating chatter marks as the drum is cut. The brake lathe should also have sharp tool bits (dull bits will not leave a good finish on the drum). When the drum is cut, the least amount of metal that's necessary should be removed to restore the drum surface and no more. This will maximize the life of the drum.
As a rule, the best drum finish is achieved with a slow, shallow cut. The smoother the finish, the better. A lathe speed of 100 to 200 rpm with a cross feed rate of no more than .002 inches per minute and a depth of cut of less than .002 inches will usually produce an ideal surface.
If the drums are turned too quickly, the tool bit can leave tiny grooves in the surface which can chew up a new set of linings as well as make noise.
One way to tel if the surface finish is in the recommended range of 80 microinches or less is to try writing your name on the friction surface with a ball point pen. If you see a continuous line of ink, the finish should be smooth enough. But if the ink line is broken up into little dots, the surface is too rough.
Resurfaced drums should be handled with care and stored face down until they are put back on the vehicle.
A common mistake that many people make is not cleaning the drums after they've been turned. Resurfacing leaves a lot of debris as well as torn and folded metal on the inside surface of the drums. Much of this debris will be knocked off by the brake linings when the brakes are first applied, but some of it will become embedded in the linings and may contribute to noisy brakes. So for best results, scrub the inside of the drums with soapy water and a stiff brush.
Once the drums have been resurfaced and cleaned, care must be used during installation to keep greasy fingerprints off of the inside surface.
On front drums with serviceable wheel bearings, the bearings must be cleaned, inspected and repacked with grease. New grease seals will also be needed along with fresh wheel bearing grease.
Before the drums are put back on, a "drum/shoe gauge" may be used to preadjust the shoes to the inside diameter of the drums. It's not necessary but can save time because if the shoes are out too far, they'll catch on the drum preventing the drum from sliding back on the brake. And if the shoes are in too far, it will take a lot of adjustment to bring them out.
Once the drums are in place, the shoes must be adjusted until they just clear the drum (little or no drag felt when the drum is rotated by hand). Adjustment will affect pedal travel, so it must be done properly.