bottle jack

I have a bottle jack ( I think 6 tons ). When I go to jack up something, it will slowing start to lower, causing the jacked item to lower also. The jack has only been used several times. Do I have to "bleed" any air out of it or should I be doing something else with it? It's definitely too late to return it. Please help!
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Sounds like the valve isn't closing all the way.
Tip it on it's side valve up, loosen the valve like your'er letting your car down and pull the piston all the way out. Now you can unscrew the valve all the way without making a big mess. Look at the valve tip closely to see if there are any signs of ununifomity. It should be perfect. Try to look in the hole for anything that might be stopping the valve from closing all the way.

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