Excessive amounts of free or entrained air in an operating hydraulic system results in degraded performance, chemical deterioration of fluid, and premature failure of components. Therefore, when a component is replaced or a hydraulic system is opened for repairs, the hydraulic system must be bled of air to the maximum extent possible upon repair completion. Hydraulic fluid can hold large amounts of air in solution. Fluid, as received, may contain dissolved air or gasses equivalent to 6.5 percent by volume, which may rise to as high as 10 percent after pumping. Free air could enter a system during component installation, filter element installation, or opening the system during repairs.
Free air is harmful to hydraulic system performance.
The compressibility of air acts as a soft spring in series with the stiff spring of the oil column in actuators or tubing, resulting in degraded response. Also, because free air can enter fluid at a very high rate, the rapid collapse of bubbles may generate extremely high local fluid velocities that can be converted into impact pressures. This is the phenomenon known as cavitation. Cavitation causes pump pistons and slide valve metering lands to wear rapidly, commonly causing component failure. Any maintenance operation that involves breaking into the hydraulic system introduces air into the system.