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Components of a Wheel Cylinder

Wheel cylinders generally come standard on rear drum brakes. Some of the older model cars and trucks have them. The wheel cylinder holds a small reservoir of brake fluid that comes from the master cylinder through a network of brake lines. By the force of pressure exerted on the brake fluid the wheel cylinder uses a piston to expand the brake shoes against the drum. Wheel cylinders can be rebuilt with a few parts and common tools. To know their inner components helps to understand how they function and what parts need periodic inspection.

Wheel Cylinder Body

The wheel cylinder body, usually cast from iron or steel, has a heavy thick construction that allows it to hold tremendous hydraulic pressure. Its hollow, smooth-bore design makes it almost a secondary reservoir in that it holds additional brake fluid supplied by the master cylinder. Resembling a regular D-sized battery in size and shape, wheel cylinders sit bolted at the top of the backing plate directly between the brake drum shoes.

Pistons

Two pistons sit inside the cylinder bore--one at each end. They have a flat surface on one side where they make contact with a cup and have a notched or recessed hole that holds a slotted brake shoe pin that slots into the brake shoe. Commonly made of aluminum for lightweight and antirust qualities, the piston receives pressure from the force of the brake pedal to expand and force the brake shoes open.

Cup Expander and Spring

A long galvanized spring sits inside the middle of the wheel cylinder bore. It has small caps welded to each end. The cap ends have convex surfaces that fit into the recessed part of the cup. When in normal mode the spring keeps pressure on the pistons to allow for instant expansion. Upon the application of force, brake fluid enters from the wheel cylinder body in between both cup expanders, allowing equal pressure to go in both directions.

Cups

The cups ride inside the wheel cylinder bore to make a perfect seal against the piston. The flat part of the cup sits directly against the piston. The recessed, or concave, part of the cup catches the force of the fluid, which drives it outward. Cups can wear or become split or brittle when the brake fluid quality deteriorates. Cups form a perfect hermetic seal and do not allow brake fluid to pass them.

Bleeder Valve

Bleeder valves, sometimes called bleeder screws, allow brake fluid to escape from the wheel cylinder or brake line. They mount on the backside of the wheel cylinder body and can be reached behind the backing plate. They screw in by the use of a tiny nut to open a small orifice. Turning them outward closes them off. Some designs of bleeder valves require a specialized wrench to reach them. Their small bore can be cleaned with a paper clip or wire.

End Boot

The end boot, or dust boot, connects to the end of the wheel cylinder. It holds the shoe pins in place, while covering the end of the wheel cylinder to guard against contaminants. A wheel cylinder leak will usually be seen at the boot end before anywhere else. They simply push on to a groove in the wheel cylinder body.