Can a homemade telescopic damper solve a pesky gate spring problem?

[sophiecentaur]

This is not actually an automotive project but the thread should suit practical petrolheads and the like, I think.
My problem is that our front garden gate needs a strong spring so that it will shut and latch itself reliably and prevent our (visiting) dog from getting out onto a fairly busy road. A while ago, I bought a conventional helical spring and it shut the gate so fast that the joints opened up and let the damp in. It is rotting fast and needs replacing.

So I decided to use a damper of some sort, ready for a new gate. The damper I made is an oil filled telescopic cylinder and the piston has a flap valve and holes in it to give low resistance one way and high resistance to the gate closing. This allows the gate to be pushed open very easily but closes slowly. The two forces it produces are very convincing in the two directions; it does the job fine. The damper is sloped, even, with the cylinder at the bottom in order to stop oil spilling out of the top end. The bottom end is (at least I thought it was) well sealed. But when the gate closes and the valve shuts, it seems that air is drawn into the bottom of the cylinder in preference to the oil flowing past the pistonn. So much so that the space above the piston gets more and more oil in it and, when the gate closes, this extra oil gets pushed out of the top. There seems to be no end to this process!

I thought I had sealed the bottom 'very well' but it still seems that air gets into the bottom of the cylinder somehow. I suppose that the pressure in the bottom must be fairly low - low enough for air tp get through screw threads etc. This is something I can chase but could there be something I have missed - like air dissolved in the oil (regular motor oil)?

It started off as a fun DIY project but now it's bugging me and the man-hours are mounting up.

 

[anorlunda]

Will an ordinary pneumatic door closer not do the job? I see them for under 8 GBP.

 

[jrmichler]

Oil will not hold that much air, especially at the speeds and forces of a gate closer. So there is another reason. Diagram, I need a diagram.

 

[sophiecentaur]

Will an ordinary pneumatic door closer not do the job? I see them for under 8 GBP.

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Yes - probably and it's true that I should just let go of this but I wanted to use something that was definitely suitable for outdoor use - not the most rational of choices. It's turned out to be a project, rather than just a gate spring.

Diagram, I need a diagram.

I know - I should have posted a diagram with the question but it's very basic - just like a motorcar front damper but without the rubber gaiter at the top. I should have done this all on paper first but I bought the tubing and a rod and used some aluminium stock from my scrap box. From that info that oil can't hold much air (which really should have been obvious to me already) there must be some leakage. I had forgotten that the pressure difference across the small gaps between the plug and tube can easily be very nearly one atmosphere - especially with the mechanical advantage of the gate. If I had welding kit, I would weld the bottom of the cylinder to the plug.
I think the solution to this would be to add a wider path to bypass the piston when it's on 'suck'. The force that the damper is producing when being pulled must actually be low because of air leaking in. As long as oil can flow easier, the pressure in the bottom space will not ever be as low as it is now and the seal should be good enough to give the oil time to flow - there is no movement down there so it's a trivial problem really. The piston is a better fit than is should be.
Thanks for the contributions guys.