Note: Steering and brakes rank high on our safety list. Whether your truck is a stock restoration project or a modern street truck build, performance means more than a hot crate motor. In this month's lesson, we'll explore brake system basics.

Hydraulic drum brakes became an industry standard by the late '30s. Designs from Bendix, Wagner, Chrysler, Lockheed, and Huck were popular. In 1939, Bendix introduced a duo-servo design that proved both efficient and easy to service. That single anchor pin, duo-servo system survives to this day.

Duo-servo Bendix-style brakes are easy to identify. They have a single, fixed anchor pin at the top of the backing plate with a dual-piston wheel cylinder mounted below the anchor. A threaded, star wheel adjuster links the lower ends of the shoes. Earlier applications required periodic, manual adjustment of the shoe-to-drum clearance. By the '60s, self-adjuster mechanisms became popular on cars and light trucks.

How Do Bendix Duo-Servo Brakes Work?
When the driver applies brake pedal pressure at the master cylinder, fluid disperses to each wheel cylinder. The curved top ends of each shoe fit against the round anchor pin. As the pistons move outward in the wheel cylinder, the shoe lining contacts the drum. The linked shoes attempt to rotate with the drum. With the vehicle traveling forward, the rear shoe stops against the anchor pin. Backing up, the front shoe stops against the anchor pin.

The obstructed rotational force now redirects, pressing the shoes outward against the drum. This additional force, referred to as "self-energizing", is beyond driver input and hydraulic pressure. When self-energizing force can apply in either direction of rotation, the system is called duo-servo.

As the vehicle moves forward, the self-energizing force is greater at the rear shoe. To keep lining wear even, the rear shoe's lining is longer and sometimes of different composition than the forward shoe. With a shorter lining on the primary or forward-facing shoe, self-energizing force is slightly less when the vehicle moves backward. Since the vehicle does not back up at high speeds, braking is sufficient in either direction.

The Bendix duo-servo design provides uncomplicated service with good parts availability. Familiar to repair technicians, these brakes also serve well at the rear of a front disc with rear drum system. For four-wheel drum brake applications, the front brake drums and lining area are often larger than the rear. This compensates for weight transfer and bias toward the front during hard braking. On a light truck without a load, weight bias is to the front. Decreasing the available braking force at the rear helps prevent rear wheel lockup under heavy braking.

Major Components
The drum brake system features a master cylinder, wheel cylinders, lined shoes, and brake drums. Master cylinders mount below the floorboard on GM trucks built to 1959 and Ford models through 1956. Other truck makes also used through-the-floor pedals. By the '60s, suspended pedals and a firewall-mounted master cylinder had become the industry standard.

Safety concerns led to the '67 mandate for a dual master cylinder. The dual master cylinder essentially splits the brake hydraulics into a front and rear system. If a leak occurs at the front wheel hydraulic circuit, the brake system still functions at the rear of the truck. Similarly, a rear leak will not cause the front brakes to fail.

Dual braking may be the most significant safety improvement in hydraulic brakes. Prior to '67 models, a simple leak at a hose, wheel cylinder, or pipe could lead to complete failure of the four-wheel braking system! While disc brakes clearly improve brake performance, the dual master cylinder dramatically enhances vehicle safety.