[Enoch]

Here is something I learnt today: If your trolley jack is playing up get some jack oil from Halfords or similar. On the barrel of the jack just forwards of where the pumping mechanism lives there will be a rubber bung - it is quite likely painted the same colour as the jack. Ease it out with a screw driver, ensure that the jack is fully lowered and the let down valve (the one you open to lower the jack) is fully open. Make sure the oil level is about 5mm below the hole that the filler plug has come out from. Any lower than this and it needs oil - add until the correct level is achieved. Pop the plug back in and with the let down valve still fully open pump the lever about 20 times. Do up the let down valve and pump the jack to it's limit, then let it down again.
I have just recovered 2 jacks that I thought were scrap by following the above method.
If my description is too hard to follow there are videos on youtube that will explain it far better than I can.
Hope it helps somebody.