Most people don't know this, but when talking about a wheel cylinder, you're actually talking about many different parts altogether. This little guide will break it down for you-figuratively, of course-to help you determine whether you are better served replacing just one or all components at the same time.
Wheel cylinder body
Because it has to handle a lot of hydraulic pressure, the body is usually cast from steel and is heavily constructed. Often, as the main body of the whole affair, it becomes critical that it fits perfectly with your particular make and model. The best guide here is you owner's manual. Alternatively, you can use matching software embedded in most online parts retailers to help you through.
These sit at either end within the cylinder. Check if they are made of aluminum. Some try to make a uniform build and make these harder steel-always go with aluminum. Considering these pistons are constantly flush with fluids, you'd want it made out of lightweight, corrosion-resistant materials. They don't need to be sturdy steel because the body protects it well enough.
Cup-expander spring and cups
This sits smack in the middle of wheel cylinder body. The spring itself is a fairly straight-forward affair and most springs of the sort are of great quality on the market-it's the cups that need tending to. These have to form a hermetic seal against the piston. Because of the pressure and stresses, these are terribly vulnerable to becoming brittle when the brake fluid quality starts deteriorating over time.
These valves allow brake fluid to escape from the wheel cylinder in the event that fluid pressure gets far too high. Of all the components, this is the one you have to least worry about because you can even clean it out with a paper clip.
So all or one
You save more on buying a cylinder set instead of replacing just one. For one thing, these are small parts and difficult to change out. The economical bet is to go for a full replacement. It saves you the bother of having to keep on changing components as each part fails in turn!