1953-1965 Corvettes with Drum Brake Systems
Reprinted from NCOA Newsletter forVettesonly
Written by: John Gunnell
Most young Corvette restorers have never struggled with drum brakes. Just looking at the array of springs and numerous other components mounted on a car with drums can be intimidating. But with some understanding of what those components do and a little practice anyone can handle servicing drum brakes with relative ease. Braking a Corvette with drums starts off much the same as slowing a disc brake Corvette. Brake fluid goes to the brakes from a master cylinder. Instead of entering a caliper, it goes into a wheel cylinder attached to the backing plate. The fluid pushes two pistons within the wheel cylinder outward, forcing brake shoes against the brake drum. Since the drum is attached to the wheel hub, the friction generated slows rotation of the wheel.
A spring sitting between the two pistons and another between the shoes ensures everything returns to its original position once the brake pedal is released. A threaded adjuster moves as the shoes wear, to keep shoes working against the brake drum. For the drum brakes to function correctly, the brake shoes must remain close to the drum without touching it. If they get too far away from the drum (as they wear), the piston will require more fluid to travel that distance, and the brake pedal will sink closer to the floor. Most drum brakes have an automatic adjuster. Teeth on the adjuster can wear and cause soppiness in the lever mechanism. If the "buttons" at the end of the adjusters (that allow them to rotate and extend) seize up, they can no longer compensate for wear.
Automatic adjusters can be serviced and lubricated. But, if there are any signs the platings or coatings have been damaged, it is a good practice to install complete new hardware set. No amount of grease will restore the corrosion-resistant surfaces.
As for lubricants, use brake lubricant-specific products. Chassis or wheel bearing greases are not designed to withstand temperatures inside a brake drum. Those kinds of grease can melt and contaminate linings. Too much grease can collect dirt.
As a brake shoe wears down, there will be more space between the shoe and drum. When you stop your early Corvette in reverse, the shoe will be pulled tight against the drum. When the gap grows, an adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, so it unscrews a little when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it keeps the shoes close to the drum until the wear is excessive.
Servicing your Corvette's rear drum brakes requires some special tools. Using needle-nose pliers or vice-grips to remove and install brake springs is difficult. They are under heavy tension and can come shooting off at a high speed directly into one's eye.
Brake spring pliers, brake spoons and brake drum adjuster tools are handy to have when replacing shoes or wheel cylinders. Brake spoons are curved to get into the adjuster window when setting the shoe-to-drum gap. Using a brake shoe hold-down spring compressor will help you remove and replace brake hold-down springs.
The most common job is replacing shoes. Some drum brakes have an inspection hole on the back so you can tell how much material is left on a shoe. You should still remove the drum to inspect all inside components. Brake shoes are replaced when the friction material wears to within 1/32" (0.8 mm) of the rivets. If the friction material is bonded rather than riveted, replace shoes when they have 1/16" (1.6 mm) of material left.
If a scored or worn brake shoe is used too long, the rivets that secure the friction material can wear grooves into the drum. A badly scored drum may be salvageable with refinishing. Disc brakes have a minimum allowable thickness, but drum brakes have a maximum allowable diameter. Since the contact surface is the inside of the drum, as you remove material from the drum brake, the diameter grows. A brake lathe is required to resurface the contact area. If the drum is scored, damaged, warped or worn, replace it.