What is a primary cause of aeration in a hydraulic system?

Aeration in a hydraulic system primarily occurs due to the presence of entrained air bubbles in the oil. This can significantly affect the system's performance, leading to issues like reduced efficiency, overheating, and increased wear on components. When air becomes trapped in the oil, it forms bubbles that can compress and expand, resulting in a condition known as cavitation, which is detrimental to hydraulic machinery.

While leaking pump seals can introduce air into the system, it is not the primary cause of aeration. Instead, it can be considered a contributing factor under certain circumstances. Similarly, having too much oil in the tank may lead to foaming or turbulence, but it doesn't directly cause aeration in the same way that entrained air bubbles do. Therefore, the most accurate answer reflects that entrained air bubbles in the oil are the main culprit behind aeration in hydraulic systems.