How can i fix my 1800Kg trolley jack?

When i jack the car up and stop, the handle wants to go back up by itself, as there must be pressure pushing that valve up, i bought it new 2 yrs ago and has only been used a dozen times, ive undone the release valve and pumped it and let the air out the oil rubber filler plug, theres a big adjustment screw on the right hand side of the handle pump, how do ya adjust this as it might be adjusted wrong. Thanks

Scott B
July 2010