This is a typical front drum...
This is a typical front drum brake. Brake fluid under pressure inside the brake cylinder pushes the brake shoes outward into the spinning drum. When the brake pedal is released, return springs pull the shoes away from the drum. This is a self-adjusting drum brake. The self-adjustment lever is controlled whenever we back up and apply the brakes. Releasing the brake pedal allows the self-adjuster spring to pull the lever down, which turns the star-wheel adjuster, moving the shoes closer to the drum.
Rear drum brakes work the...
Rear drum brakes work the same way as the fronts, with the wheel cylinder applying pressure to the shoes, which moves them against the spinning drum. Rear drum brakes differ in the use of a parking brake mechanism. This mechanism consists of the parking brake lever, link, link spring, and the cable.
Here are two illustrations...
Here are two illustrations detailing front and rear drum brake wheel cylinders. Each brake cylinder has two cups, two pistons, a return spring, and two boots. Cylinder sealing happens thanks to the cups, which contain the pressure. Pistons transfer pressure to the pins and shoes.
Drum brakes have been around since the beginning of the 20th century and there have been many variations along the way. Most classic Mustangs (1965-73) were equipped with drum brakes. All 1974-93 Mustangs were fitted with front disc brakes. Mustangs after 1993 were fitted with four-wheel disc brakes. This should tell you something about braking technology. Drum brakes are old technology. Front disc brakes have been available as an option on the Mustang from day one.
A drum brake is more complex than a disc brake because there are more parts involved. The wheel cylinder receives hydraulic pressure from the master cylinder when we apply the brakes. Wheel cylinder cups and pistons move outward against the C-shaped shoes lined with friction material. The friction material contacts the rotating drum, which is an integral part of the axle and wheel. This friction brings the drum, wheel, and tire to a stop. Three return springs pull the shoes away from the drum when the pedal is released. A star-wheel adjuster between the two shoes at the bottom adjusts the shoes in or out.
Self-adjusting drum brakes employ even more parts. All Mustangs prior to 1994 have self-adjusting drum brakes. The self-adjuster consists of a cable, pivot hook, and return spring, and it only works when backing up and applying the brakes. As the shoes contact the drum while backing up, the pivot hook (tied to the cable, shoe, and return spring) moves away from the star-wheel adjuster, then returns, moving the star-wheel adjuster, thus moving the shoes closer to the drum. When properly maintained, self-adjusting drum brakes work quite well, but they quit working whenever the pivot hook is damaged. They also quit whenever the star-wheel adjuster ceases. A good rule of thumb is to inspect the self-adjuster for proper operation any time you repack wheel bearings or rotate tires. Get familiar with your drum brakes. They can save your life.
The parking brake holds one of the rear brake shoes against the drum to hold the vehicle. We pull on the T handle, press the parking brake pedal, or pull up on the handle, which pulls a cable tied to the rear drum brake shoes.
Drum brakes deserve more credit for braking effectiveness than we give them. Drum brakes offer us more braking surface area than a disc brake. They can be very effective, especially at the rear axle. The problem with drum brakes is their effectiveness in some situations. Theyre downright dangerous in wet weather. If they get soaking wet in high water, for example, they will not stop a car because water between the shoe and drum causes the friction material to hydroplane, just like a tire does on wet pavement. Water keeps the friction material from contacting the drum and stopping the vehicle.
Drum brake maintenance should include a regular inspection schedule. Pull the drums and inspect them every 5,000 to 10,000 miles. Lubricate the star-wheel adjuster with a thin, high-temperature grease. Examine the wheel cylinder rubber boots for evidence of leakage. If there is brake fluid inside the rubber boot, rebuild or replace the wheel cylinder.
If your drum brakes tend to be noisy, when are they noisy? If they squeak when youre applying brake pedal pressure at a standstill, there isnt enough lubrication between the shoe and the brake backing plate. If they squeal during application with the vehicle in motion, the shoes and drums are glazed and should be serviced. Drums and shoes become glazed whenever there has been heavy braking and severe heat. They can also become noisy whenever the drum has been turned too many times and is out of tolerance. Then the shoes have to travel beyond their normal travel distance, contacting the drum unevenly or not at all.