The majority of hydraulic systems failures are due to contaminated, or ageing hydraulic fluid.
Contamination causes degradation of fluids and hydraulic systems failure for several reasons. Firstly, it can cause increasing internal leakage (which lowers the efficiency of cylinders). Secondly, contamination decreases the ability of valves to control flow and pressure, thus wasting power and generating excessive heat.
Furthermore, contamination can cause parts to stick due to sludge or silting, or even seize the assembly when large amounts of contaminants accumulate in the clearances.
Sources of contamination can include the manufacturing process, hydraulic fluids, environmental exposure, system wear and servicing. The most common contaminant is water.