Drum Brake System: Description and Operation

COMPONENTS

GENERAL DESCRIPTION
The drum brake assembly has a self shoe clearance adjusting system so that drum-to-shoe clearance is maintained appropriate at all times.

NOTE: Replace all components included in repair kits to service this drum brake. Lubricate parts as specified.

WARNING: If any hydraulic component is removed or brake line disconnected, bleed the brake system. The torque values specified are for dry, unlubricated fasteners.

REAR BRAKE OPERATION
With the general drum brake type, when the brake pedal is depressed, two pistons in the wheel cylinder force the brake shoes outward, restraining the turn of the drum.
The more the brake shoes get worn, the longer distance the pistons must move. As a result, the brake pedal travel increases. Then the shoe clearance must be adjusted by the shoe adjusting screws. Thus periodical adjustment is required for the drum brake type in general.
This rear brake is provided with a self-adjusting system which automatically adjusts the shoe-to-drum clearance (pedal-Pedal - to wall clearance) caused by such brake shoe wear.





CLEARANCE CORRECTION
In each rear wheel cylinder, pistons, piston cups, and a piston spring (1) are installed. When the brake pedal is depressed, fluid pressure is applied to the inside of the chamber on the pistons (2) and (3).
Being actuated by this pressure, the piston (2) moves to the left (piston (3) moves to the right) in the figure and presses the brake shoe against the brake drum, thus producing brake force.





At this time, the distance the brake shoe moves is "B", that is, the distance that "A" (the end of the long hole made in the brake shoes web) moves till it contacts the lever (1) which is fitted in the long hole.
When the brake pedal is depressed, the piston and brake shoe move toward the brake drum side by the aforementioned distance "B" and "A" of the brake shoe web contacts the lever (1).
As the brake shoe gets worn and the brake shoe clearance becomes larger, the force applied to the lever (1) at the time of such a contact becomes larger. When it exceeds 10 - 12 kg (22 - 26 lbs), the "A" of the brake shoe web moves the lever (1) as much as the amount of the brake shoe lining wear toward the direction as shown with an arrow in the figure. Thus the shoe is forced against the drum and the brake force is produced.





The distance the lever (1) moves corresponds to the amount of wear. In accordance with the lever (1) movement, the fan shaped ratchet (2) also moves, for they are assembled as a unit. The lever (1) and ratchet (2) remain in the positions as they moved until the shoe-to-drum clearance becomes even larger.
When the brake pedal is released, the brake shoe is allowed to move back by the amount of clearance "B" by means of the return spring. In this way, the brake shoe-to-drum clearance is automatically adjusted constant every time the brake pedal is depressed.

The brake shoe-to-drum clearance "B" corresponds to 0.36 - 0.54 mm (0.014 - 0.021 inch) in terms of the brake drum diameter A <-> A'. And the amount adjusted by one notch of the ratchet corresponds to 0.18 mm (0.007 inch) in terms of the brake drum diameter A <-> A'.

The spring provided in the wheel cylinder prevents the piston from moving back more than the specified brake shoe-to-drum clearance.