What condition could cause the controls of a hydraulic system to feel spongy?

A spongy feel in the controls of a hydraulic system is predominantly caused by air being present in the system. When air enters the hydraulic fluid, it compresses under pressure and can lead to a lack of responsiveness in the controls. This is because hydraulic systems are designed to transmit power through incompressible fluid; when air is mixed with the hydraulic fluid, it disrupts this process. The presence of air creates a cushion effect, making it feel as though the controls are not operating smoothly or have a 'spongy' characteristic.

Understanding this is vital for maintaining hydraulic systems because air can enter through various means, such as leakage or improper servicing. Addressing air in the hydraulic system typically involves bleeding the system to remove the trapped air, ensuring optimal performance of the hydraulic controls and machinery.