Trolley jack wierd fault

[Dal5band]

Got a 2.5 tonne Halfords jack about 2yrs old. When i pumped it up and pressed on the arm, the pump rod started moving. Every time I pressed on the saddle, the rod would jump up and would go down under its own weight after i took my foot off the saddle. What's causing this ? If i stand onthe saddle, it holds firm after the initial jerk.
Any ideas ?

[Evolved]

Not really sure what you mean. Can you try to explain it better, or just post a vid maybe.

[GreenV8S]

Sounds like one of the one-way valves has failed.

[Dal5band]

That sounds right some sort of non-return valve is allowing back pressure. I.e when you press onthe lifting arm, the pump rod moves. This never happened on my old smaller trolley jack in over 10 years.
What's the solution ?

[GreenV8S]

Dal5band said:

What's the solution ?

Dismantle, find the faulty valve, repair or replace it.

[Dal5band]

If all else fails read the instructions ! I downloaded the manual and the first option is to clear any dirt that may have lodged in the non-return valve. This worked and its working fine now. There is no need to dismantle anything :

With the lifting arm at its lowest and the release valve closed, put a foot on the front wheels and lift the jack arm up by hand as far as it will go. This makes the hydraulic fluid rush into the lifting piston (ram) , the rush of fluid clears any debris from the valve. Lower the arm by opening the release valve. Repeat if necessary. This may also help clear air bubbles.
If this dosn't work then there is a more serious problem with the various valves - probably need a specialist or a new jack.