This has been discussed before somewhere else in GJ. And I am afraid I will have to disagree somewhat. Sorry about that.
While Hiball maintains that the rubber plug will vent under pressure, I have to state that a plug in good condition (not hardened or worn out), and also, the hole itself in good condition (not rusted or oval or enlarged somehow) will seal good enough that it prevents air from escaping the reservoir and it will also allow a vacuum to form inside the reservoir. Also, threaded metal plugs, with a sealing washer underneath, will seal so well that not air will escape. That this happens more or less, depends on the amount of air entering the reservoir and the causes of this air entering the reservoir, like worn o-rings somewhere else on the pump. A jack with good o-rings should retain, or create, less air pressure inside the reservoir.
Understand that these plugs have to seal well enough to prevent oil leaking out from the jacks while these are transported from the manufacturer to the dealer. If the jacks are stored upside down, for the duration of the voyage and the storing in the dealer store, a leaky plug would result in an empty oil reservoir.
Also, rubber plugs usually have a mushroom shape on the part that goes inside the reservoir (the bottom). When the air pressures pushes the plug, the wider part at the bottom of the plug gets pushed against the oil hole, increasing the sealing ability of the plug. Only if there is any issue with the plug condition or the hole condition, will the plug vent the air pressure. But not because that is what it is supposed to do, but because it is leaking.
Jacks like the Hein Werner 93642 gets shipped from the manufacturer with a solid threaded plug and the owner is required, in the instructions, to replace it with the supplied breather plug so the jacks works OK. Why does Hein Werner ( Shinn Fu ) urges the owners to change the plug? Because they know that there can be issues with either vacuum or air pressure inside the reservoir tank. Also, if no breather plug was supplied, the instructions required that the solid threaded plug be backed a turn or so, so the pump could release or allow air to enter as needed. That most jack manufacturers do not offer a breather plug, even when necessary, is just a way to save money. And anyway, most person never read instructions and the plugs would never get replaced.
But no one has to believe this by faith. This is not a Jim Jones or David Koresh thing that you have to believe in it blindly. Anyone with a jack, and assuming the plug is in good condition, can test this by himself. Just cycle the jack a few times (all the way up and down) and then push the plug aside a little and see if air comes rushing out. Either push it with your finger or use a rounded tip flat blade screwdriver so as to not damage the plug.
If the plug is in good condition, more often than not, some air will come rushing out when the plug is pushed aside. If you see oil coming out around the plug, or even leaking air, without having to push it aside, either you plug is not in good condition or the fill hole is enlarged somehow.
Please, anyone with a jack, test this before forming his own opinions about this issue.
I understand that Hiball might disagree about this strongly, but still I have to disagree with him because I feel the need to give the other side of the issue. I am not doing this to bother anyone nor I disagree just for the sake of disagreeing.
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