This Bendix drum brake layout...
This Bendix drum brake layout became an industry standard. Pre- and post-war trucks use Bendix-, Chrysler-, Lockheed-, Wagner-, or Huck-style brakes. By the end of the '50s, the Bendix design dominated American light trucks. Self-energizing Bendix duo-servo brakes have a dual-piston wheel cylinder, a single (top) anchor pin and an adjuster link between the shoes. Bendix duo-servo brakes with self-adjusting mechanisms are found on disc front/rear drum systems of modern trucks.
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.
Airborne asbestos is a known...
Airborne asbestos is a known carcinogen and cause of lung disease. Once wetted, asbestos is no longer airborne. Here, the nozzle of a brake parts washer has flowed aqueous solution between the brake drum and backing plate before removal of the drum. If you work on an older truck chassis with its original brake shoes, beware of the safety risk. Respirators must be rated for asbestos dust protection.
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.
After flushing with the wand...
After flushing with the wand and brush, old shoes get placed in the washer's containment basin. You can roll back the dust boot edge to check for signs of fluid leaks. Fluid pressure can dislodge the wheel cylinder pistons and dust boots. If the hydraulic system does not need service, a set of wheel cylinder clamps will retain the pistons and prevent fluid contamination. This cylinder will be removed and disassembled.
The brake drum is full of...
The brake drum is full of dust, too. In the basin, aqueous solution washes away dust and keeps it from going airborne. This popular Ammco 1450 brake washer has an air-motor pump, hoses, a wand, and a brush. A filter sock in the drain and a dense filtration system trap asbestos and other contaminants. Consider brake dust containment and disposal before working on your brake system. Do not use a conventional shop vacuum on brake dust. Asbestos will pass right through the filter and discharge into the air. When replacing brake shoes, request non-asbestos lining if available.
It doesn't get more authentic...
It doesn't get more authentic than these vintage GM ball-type wheel bearings! Through 1961, Chevrolet and GMC light trucks use ball-type front wheel bearings. Ford, Dodge, Studebaker, and I-H each use Timken-style tapered roller bearings. Ball bearings require care and special service procedures. Vintage GM truck owners often convert to tapered roller bearings. These ball bearings, however, will be serviced and set to factory specifications.
Rear brakes were disassembled...
Rear brakes were disassembled while thoroughly wet, the best way to contain brake dust. Note the emergency/parking brake cable and axle shaft flange. Bendix drum rear brakes contain the parking brake. Parking brakes are mechanically actuated by a cable. Look carefully for seal leakage at the axle shaft. (Gear oil will ruin fresh brake lining!) Check for radial play at the axle shaft bearing. If wear is evident, replace the axle shaft bearing and seal.
Note the glaze, scoring, and...
Note the glaze, scoring, and light grooves on this rear brake drum. After dust cleanup, measure the diameter of the drum at several places. Compare these measurements to the maximum allowable size. If the drum can be resurfaced within factory diameter limits, do so. Operating a drum beyond the maximum recommended diameter can cause poor braking and lead to drum failure. Check for hard spots, warp, and deep scoring grooves. A brake shop or automotive machine shop can resurface drums.
This is a half-century-old...
This is a half-century-old wheel cylinder. Corrosion, rust, deep pits, and wear suggest replacement with a new cylinder. Years ago, honing was an acceptable practice for very light surface blemishes and stains. By the early '70s, the practice of honing gave way to removal of minor surface stains with crocus cloth. Use alcohol (denatured or isopropyl) to clean the cylinder. Never use mineral-based oils or solvents near rubber parts! If working around grease or oil, wash hands thoroughly with soap and water to prevent contamination of rubber.