What is a Car Fluid Reservoir?

Dan Cavallari

A car fluid reservoir can refer to any container within an automobile that holds a liquid used for the normal functioning of that automobile. A fluid reservoir might hold power steering fluid, transmission fluid, or even windshield wiper fluid. These reservoirs are usually located within the engine compartment of the automobile, though not exclusively so. Some racing cars have fluid reservoir units inside the cab of the automobile for quick adjustments and level readings, though the vast majority of reservoirs in an automobile are either in the engine compartment, under the vehicle, or otherwise hidden from sight.

Race cars may include fluid reservoir units inside their cabs to allow for quick adjustments and level readings.
Race cars may include fluid reservoir units inside their cabs to allow for quick adjustments and level readings.

In most cases, a fluid reservoir is made from a type of plastic that will not break down or corrode when in contact with the fluids inside it. Some of the fluids a reservoir will hold can be caustic, meaning the fluid can break down certain materials such as metals. The types of plastics used to make a fluid reservoir will resist breakdown when exposed to caustic fluids; plastic reservoirs are also less expensive than other materials, so if the reservoir breaks or otherwise cracks, replacement costs will be low.

The fluid reservoir most car owners are familiar with is the windshield wiper reservoir. It is one of the larger reservoirs within the engine compartment, and it holds the cleaning solution that is sprayed out through jets onto the windshield. The reservoir is mounted with bolts to a part of the engine compartment; a small opening at the bottom of the reservoir is attached to a pump system that is in turn attached to a hose that leads to the fluid sprayers. The motor often features a sensor that will indicate if the reservoir is close to being empty. The top of the reservoir features an opening through which new fluid can be poured.

Other important reservoirs include the brake fluid reservoir and the power steering reservoir. Each of these small tanks holds a hydraulic fluid — most often a type of oil with a thicker consistency than water — that feeds the respective hydraulic systems. Brake systems can be bled through the reservoir, and the power steering fluid can be added to the steering system through the reservoir. If either of these reservoirs are low on fluid, the car owner will know a potential problem has arisen in the systems. When bleeding brake systems to ensure no air bubbles are in the brake lines, the reservoir is uncapped to allow the air bubbles to escape.

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