What would cause the hydraulic system to lose pressure?

The hydraulic system losing pressure is most commonly caused by a leak within that system. Hydraulics operate on the principle of incompressible fluid under pressure, and if there is a breach in the system—such as a cracked line, a damaged seal, or a loose connection—the pressurized fluid will escape. This escape leads to a drop in pressure, which can compromise the functionality of critical components that rely on hydraulic power, such as flight control surfaces or landing gear.

Other options do not lead to a loss of hydraulic pressure directly. A full reservoir, for example, ensures that there is enough fluid to maintain pressure rather than diminish it. An engine fire suppression system, while it may have a connection to hydraulic components, does not itself cause a pressure loss in the hydraulic fluid. Fuel contamination affects fuel systems and combustion, not hydraulic pressure. Thus, a leak is the definitive cause of pressure loss in a hydraulic system.