The Nissan drum brake wheel cylinder performs a similar function as the brake caliper does; as it also uses force generated by hydraulic pressure and converts it to mechanical motion. This function is accomplished with the pistons that move out as hydraulic pressure is forced into the drum brake wheel cylinder. Nissan drum brake wheel cylinders can only be found on the rear wheels of a vehicle made by this manufacturer. This braking component is found on the upper center portion of the rear backing plate of the drum for each wheel. Inside of the drum brake wheel cylinder are two pistons that move outward as the pressure builds. They push against the drum brake shoes which are then forced against the inside of the drum of the brake causing friction to occur and the wheel to stop moving. This is how the drum style of braking system works on your Nissan. The only problem that can occur with the drum brake wheel cylinder is when the seals on the outside of the piston in the cylinder itself wear out and allow for the hydraulic fluid to leak out causing a brake failure on your Nissan. This problem can be noticed when there is a puddle of brake fluid on the pavement and on the inside of the drum. When replacing the drum brake wheel cylinder, the first step would be to remove the brake line. This might be difficult since it has been in place for many years. Once removed the entire cylinder can be removed and the new one put into place. With the air bled from the braking system, your Nissan will then be ready to be operated on the road safely.