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Sounds to me like the check ball is stuck. You need to remove the pump arm and the retainer nut on the pump shaft, this will allow you to remove the pump shaft. There should be a spring and steel ball under the pump shaft housing which should also unscrew from the base of the jack (depends upon brand and style as to where this is located). If this ball sticks in the closed position, it will not allow the pump to build pressure on the cylinder rod because it cannot draw fluid from the reservoir (the ball allows the oil to flow only in one direction). You will still see the fluid flowing through the release screw but all you are doing is moving the small amount around that is trapped in the pump itself. The most common cause of the sticking ball is rust if the jack is not used often. If the ball and or ball seat and or spring is damaged from rust, throw the jack away and get a new because it will no longer be safe to support a load. If the ball is just gunked up with crud, you can clean it with a rag and some clean jack oil and re-install the parts. While you have it apart, drain all the old oil and replace it with new after putting the rest of the jack back together. Some jacks use a floating piston or piston and spring instead of a ball and spring for the check valve. You will find the same similar set-up as above except you have to ensure that you replace the piston exactly the same direction as it was removed. In either case, if anything looks to be damaged from rust or dirt, replace the jack. Getting hurt or killed to save $50 on a jack is not a good thing.
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