[159] Automatic bleed valve
When the cylinder is
retracted, the piston of the bleed valve is closed. It is lifted
as the piston rod advances. Air can then escape via the bleed
hole until the hydraulic fluid reaches the piston and pushes it
upwards. In the forward end position, the piston is pushed fully
upwards by the hydraulic fluid and thus provides an external
seal.
Bleed valves should be fitted at the highest point in a piping system, since this is where any trapped air will collect.