Drum Brake Wheel Cylinder

What a wheel cylinder does

In a drum brake, the wheel cylinder sits at the top of the brake backing plate. When you press the pedal, hydraulic pressure pushes pistons outward and moves the brake shoes against the inside of the drum. Rubber cups seal the fluid inside, and over time those seals can fail.

Signs of a failing wheel cylinder

• Brake fluid on the inside of the wheel, backing plate or drum.
• A soft or sinking pedal, or a need to pump the pedal.
• Brake shoes soaked in fluid, which lowers braking power and may cause grabbing.
• A brake that drags or does not release, from a sticking piston.
• Lower fluid level in the master cylinder reservoir with no other visible leak.

Choosing the right cylinder

Wheel cylinders are matched by bore size, mounting style and fitting position, so a cylinder that looks similar can still be wrong. Product details often list the model and year, such as a CJ-5 or CJ-6 or a ZJ Grand Cherokee, the axle (for example Dana 35), the drum size (such as 9 x 1-3/4 inch) and the position. Some cylinders fit either side, while others are specific to the driver or passenger side or the front or rear. Use the factory part number or measure the existing cylinder's bore if you are unsure.

Replace as a set of parts

Replace the cylinders on both wheels of an axle, and inspect the shoes, drums, springs and hardware at the same time. Contaminated shoes should be replaced because fluid cannot be cleaned out of the friction material. A hone-and-rebuild is an option on some cylinders, though replacement is usually simpler and more reliable.

Installation notes

Brakes are safety-critical. Use brake cleaner, not compressed air, on dusty drums, and avoid breathing brake dust. Tighten the brake line fitting and mounting bolts to the manufacturer's torque. After installation, bleed the entire system with fresh fluid of the type in the owner's manual and test the pedal firmly before driving. If you are not experienced with brakes, have a qualified shop do the work. Confirm fitment for your exact year, make and model.

Other drum brake parts worth checking

While the drums are off, look at everything. Inspect the brake shoes for thickness, glazing and contamination, and replace them in axle sets. Check the springs, adjusters and retainers for rust or weak tension, and replace hardware kits where practical, since small parts wear out together. Measure the drums for scoring and for diameter against the maximum marked on them. Look at the brake lines and flexible hoses for corrosion or swelling, because old rubber hoses can cause dragging. Clean the backing plates and check the axle seals for leaks, because gear oil on the shoes ruins them. Lightly lubricate the contact points on the backing plate with a high-temperature brake lubricant, avoiding the friction surfaces, and adjust the shoes to the specified clearance so the pedal feels firm.

Common mistakes include reusing damaged bleeder screws, over-tightening the line fitting until the cylinder cracks, and getting grease on the shoe lining. Make sure the bleeder screw faces up, since air rises, and confirm that the pistons move freely before final assembly.