You never see them, but you always need them. Behind those rear brake drums they lead thankless lives, and hopefully are always ready to slow your 3,500-pound musclecar down. In a world of four-wheel ABS disc brakes, a good percentage of modern and vintage cars still rely on drum brakes. A properly working duo-servo drum-brake system teamed with good front disc brakes can provide reliable stopping performance. But left unchecked, it can deteriorate and become dangerously unpredictable.
The duo-servo type of brake system (used on GM vehicles for decades) uses a leading (primary) and trailing (secondary) brake shoes, one wheel cylinder (mounted at the top between both shoes), and a lower connection, or star adjuster. A comparatively smaller friction area identifies the primary (forward facing) and the secondary (rearward facing) shoes. As the wheel and brake drum rotate and the brakes are applied, hydraulic force expands the wheel cylinder, forcing the top sides of both brake shoes outward. As the drum continues to turn, both shoes moving outward (at the top) and the resulting downward friction grab the primary shoe and pull it away from the wheel cylinder. The force exerted on the primary shoe is transferred in an expanding rotation to the secondary shoe, which multiplies friction. These dual actions (the hydraulic force of the wheel cylinder and the self-energizing effect from the shoes) are called a duo-servo system.
By removing the brake drums, problems such as worn shoes, leaking wheel cylinders, or scored drums are easy to spot. If the drum's diameter is in question, many auto-parts stores can measure them. If you find that your system needs repair, it typically takes about an afternoon to get your drum brakes back in order.