The presence and location of brake calipers can vary depending on the make and model of ATV. If a caliper is present, it will always be found straddling a rotor, also known as a brake disc.
While many ATVs have four brake calipers, one for each wheel, it is not uncommon for an ATV to have only three calipers. Typically, a brake caliper straddles each rotor in the front, which is mounted to the wheel hub. In the rear, a rotor may be mounted to the axle. This configuration can be seen on a Yamaha Warrior.
It is also not uncommon for an ATV to have four individual brake calipers that are not at all interchangeable; the Yamaha Grizzly is an example of this practice. There are also a number of side by side UTVs which have two front brake calipers but only one caliper in the rear, which is located on the input of the differential. For example, this brake caliper configuration can be seen on the 2014 ODES Dominator.
The size of brake calipers varies as widely as the makes, models, and sizes of an ATV. Some of the them are colored and some of them are bare metal. The best way to differentiate a brake caliper from any other ATV part, is to understand it's function. Regardless of location, a brake caliper will always straddle a rotor. Brake calipers are used only in applications that require both brake pads and brake fluid. Therefore, a brake caliper will always have a shape conducive to the curvature of a rotor and brake pads, as well as possess a brake line (or at least an attachment for a brake line).
Brake calipers also feature at least one caliper piston, although this is another factor which varies by application. Most ATVs have just one caliper piston, but this is not a hard and fast rule. Generally, the larger the application, the larger the brake pads, which equates to the need for additional pistons. For example, a 2008 Yamaha Grizzly 700 has four brake calipers, with one piston each. This can be seen mounted in the photo above. The 2014 ODES Dominator has three brake calipers with one piston in each the front calipers and four pistons in the rear caliper. The rear caliper is shown unmounted in the photo below.
When the brakes are applied (either by hand or foot brake), brake fluid is pushed from the master cylinder to the caliper piston. The force of the brake fluid causes the piston to apply pressure to the brake pads, which then squeeze the rotor. The friction and heat created by the brake pads on the rotor creates a drag on the rotor. In instances where the rotor is mounted to the wheel hub assembly, the drag on the rotor causes the assembly to slow. This in turn slows the turning of the wheels, which slows the vehicle. In instances where the rotor is mounted to the rear axle or differential, the drag on the rotor causes the drive shaft or axle to slow. This slowing causes both wheels and thus, the vehicle, to slow.
While brake pads are common wear items, the brake caliper housing is incredibly low maintenance and in most circumstances, will last the life of the ATV. In the instance that a brake caliper component is damaged during removal or installation, aftermarket rebuild kits are available in the $10-$30 range depending on the application. These rebuild kits usually include bleeder valves, rubber components, and pad hanger pins. While aftermarket rebuild kits are readily available for most models, rebuilding a caliper is a task which might be better left to a professional. Removing the caliper piston takes some specific knowledge and experience. In the wrong hands, this process can cause severe injury or worse. As with all parts, depending on the make and model, it may be cheaper to buy a new caliper than to attempt to have it rebuilt.
Because of the importance of the brake calipers function, as well as the difficulty with testing prior to purchase, it is not recommended to buy a used brake caliper.