What causes air in the hydraulic system and how can it be bled ?

Air enters hydraulic systems through the suction line of the hydraullic pump, and it can be overcomed by well sealing the suction line, and maintane hydraulic oil level in hyddraulic tank

There is no special cause of air in hydraulic system , its because when we open , dis.assembled or remove any component of closed hydraulic system the air take place between the parts and cause multiple hard-dales like air lock, pressure not buildup , fluctuation, and some kind of performance less operation.

Air bleeding methods are different for system to system , it may be via giving drive manually or electrically to the system and chose one point in pressure line to bleed or may be bleeded via bleed lines or automatically bleeded via tank or reservoir.