Jacks

A jack comprising:

• a cylinder;
• a piston mounted to slide in the cylinder;
• a first chamber defined within the cylinder on a first side of the piston;
• a second chamber defined within the cylinder on the other side of the piston;
• said second chamber having an end wall facing said piston;
• a discharge duct in said end wall connecting the second chamber to the outside of the cylinder;
• a pusher having a main cylindrical surface mounted to slide axially in the piston, the pusher having an end face facing said end wall of the second chamber, biasing means urging the pusher partially out of the piston toward said end wall, said end face of the pusher being arranged to bear against the said end wall as the piston approaches the end of a stroke towards said end wall to block direct communication between the second chamber and said discharge duct, aperture means arranged in the pusher for establishing secondary communication between the second chamber and the discharge duct through said pusher, the end face of the pusher having a diameter that is less than the diameter of said main cylindrical surface so that the effect of the pressure of the fluid contained in the second chamber creates a resultant force on said pusher opposite to the force of said biasing means, said resultant force being sufficient when said end face is bearing against said end wall and the pressure reaches a predetermined value to move the end face of the pusher out of contact with the said end wall to move the pusher and establish direct communication between the second chamber and the discharge duct to lower the pressure in said second chamber.