What is Wheel Cylinder Cup and Why Do We Use Them?

Hydraulic drum brake system

A wheel cylinder is a component of a hydraulic drum brake system.[1] It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact with the drum and stop the vehicle with friction.[2] The wheel cylinders are usually connected to the shoes with small bird-beak shaped rods. Wheel cylinders were first invented by Bendix in .

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It is very similar to a slave cylinder and functions in much the same way, internally consisting of only a simple plunger. On older vehicles, these may begin to leak and hinder the performance of the brakes. They are, however, normally inexpensive and relatively easy to replace.

The wheel cylinder consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out, pushing the shoes into contact with the drum. Some designs use two single piston wheel cylinders, one at the top of the drum and one at the bottom, each connected to one brake shoe.[3]

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Wheel cylinders must be rebuilt or replaced if they show signs of leaking. Wheel cylinders used to be made of cast iron. However, they were more prone to rusting and aluminium is now the preferred material.

It has a cylinder, two pistons, two rubber cups and a spring. The fluid presses against the pistons that move outward in the cylinder. When the pistons come closer, the liquid is forced into the master cylinder, the spring between the two pistons holds the rubber cups in positions.

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I posted this in the wanted forum a couple of weeks ago but have gotten no response so thought I would run it by you guys and gals. My new LEFT front wheel cylinder (on the Darrin) leaked from the large bore. A few weeks ago I disassembled my old wheel cylinder and found brass cup expanders that were not present in the new cylinder. I cleaned them up and installed them in the leaking cylinder and solved the problem. Fast forward to a couple of weeks ago and there was a puddle of brake fluid under the tire from the RIGHT front wheel cylinder. So I disassembled the old cylinder from that side but there were no expanders. So the hunt continues for a large bore expander (1 1/18") and a small bore expander (1"). Any help in locating these items would be greatly appreciated. Thanks, Steve , Darrin #3  Thanks for the reply. NAPA has been my go-to parts house for over 40 years. They failed me. The guy I bought the wheel cylinders did not respond to my inquiry. I broke down and made them out of sheet copper. I know that they might loose their form over time due to the softness of the material but that was all I could think of to get back on the road. Steve