The Importance of Drum Brake Self Adjusters
Drum brakes have return springs to return the shoes to a rest position and hardware to hold the shoes to the backing plate. The shoes return to a fully retracted or rest position when the pressure to the wheel cylinder is released. There is no compensation for lining wear.
In some cases, the master cylinder may contain residual check valves to prevent air from being drawn past the wheel cylinder cup seals when the shoes are retracted by the return springs.
The Bendix/Wagner self-energizing brake has a single anchor pin located above the wheel cylinder. The primary shoe is identified by its length and position on the shoe. The lining segment is shorter than the lining segment on the secondary shoe and is positioned toward the adjuster or heels of the shoe. When the brake is applied, the master cylinder applies hydraulic pressure to both wheel cylinder pistons.
The primary and secondary shoes engage the friction surface of the drum. The wrapping motion of the primary shoe transfers pressure through the adjusting screw driving the secondary shoe against the anchor pin and friction surface of the drum.
The secondary wheel cylinder piston is held in its rest position as the secondary shoe is driven against the anchor pin.
The adjuster mechanism is attached to the secondary shoe. It is a mechanical device that is made up of the three components.
First, an adjusting screw is a threaded device like a nut and bolt. The head of the bolt is a notched wheel with a cylindrical pin. A washer and slotted cap fits over the pin and engages the web section of the secondary shoe. The nut is also a slotted cap that threads onto the bolt and engages the primary shoe.
Second, a lever moves on a pivot to engage and turn the notched wheel.
Third, a cable or linkage is attached to the lever and the anchor pin to move the lever at its pivot point. There are springs and retainers to hold the mechanism in place.
The adjustment takes place when the vehicle is in reverse and the brakes are applied or when the parking brake is engaged. The cable or link attached to the anchor pin pulls the lever mechanism based on the movement of the secondary shoe.
When the brakes are applied, the lever cannot move the adjuster screw because the brake pressure is being used to apply the shoes. The adjuster lever mechanism must store the adjustment and turn the notched wheel when the brake is released.
There are two types of Bendix/Wagner self-adjusters. Type 1 uses a cable attached to the anchor pin and hook that is attached to the adjusting lever. A guide holds the cable in place. A spring connects the lever to the primary shoe to contain the adjuster. Type 2 uses a cable attached to the anchor pin with a spring and guide that attaches to the lever.
Type 2 uses the same cable guide as Type 1. On Type 2, the lever is connected to the secondary shoe with a pin and return spring. A spring connects the primary and secondary shoe to contain the adjusting screw.
On the Delco Moraine type, the adjuster uses a lever assembly, linkage and adjusting screw. The lever assembly is made up of the adjusting lever with spring-loaded link. The linkage is attached to the anchor pin and spring-loaded link of the assembly. The lever is attached to the secondary shoe at the shoe hold down. A lever return spring is placed between the lever assembly and shoe. A spring connects the primary and secondary shoe to contain the adjuster.
Dual Servo brakes use two primary shoes and two anchor pins. The pistons of the wheel cylinder apply both of the shoes. The adjuster is located under the wheel cylinder. The adjuster uses an adjusting screw and notched wheel and lever in the same way as the Bendix/Wagner unit. A lever is attached to one of the shoes to adjust the notched wheel. The lever is usually part of the parking brake assembly.
There is no pressure on the adjusting screw when the brakes are applied, allowing the lever to turn the notched wheel. A return spring is used to contain the adjuster as with the Bendix/Wagner type.
The lever and notched wheel are in contact and move against each other every time the secondary shoe moves off the anchor pin. A groove can be worn in the lever and the notched wheel will also wear. A groove worn in the lever can prevent the adjuster from working properly.
In a disc/drum application, the automatic adjusters play an important role in maintaining proper brake balance when the shoes are seating to the drum. The linings on premium replacement shoes are eccentrically ground to produce a clearance at the shoe’s heel and toe. For the first 100 miles or more, the adjuster will be the most active and keep the brake pedal at its proper height. After the shoes have seated, the adjuster will turn the notched wheel in small increments. Replacing the adjusters can ensure proper brake application and pedal height.