Whats an adequate fill plug for a floor jack?

[sofasurfer]

I was removing the fill plug on my floor jack when it went "plefff" out across the front yard. Needless to say its gone. There was pressure behind it (does no good to know that now) but how much. Will any plug work? I found some black rubber "corks" at the hardware that may work. Heres a post on the subject with different kinds of plugs.

Viewing a thread - Where to get little rubber plug for a hydraulic jack

One guy says a plug has to allow "breathing" My old plug had no breather hole in it. I believe that this should be a closed system, not a breather.
I don't mean to over think this, I'm just worried that if my "junk drawer" plug pops out the jack could come crashing down.
BTW, its a MVP Super-lift 3 ton jack. The plug hole is 5/16 inch.

 

[rsanter]

Do you know what the old plug was like?
Was it just a bolt/screw with a seal on it?
If so I would do my best to replicate that

 

[mikey03]

I looked into this last month when I thought about repairing one. Maybe I’m wrong but what I took from it is when you pump a hydraulic jack your pushing air in which pushes the fluid further into the system and like car breaks you are pushing the thing with the force of the incompressable fluid.

When you release it the air got to go somewhere so you’re releasing the air. There’s a ball bearing with a spring that makes it one way.

So idk if there’s some of these with no breathing in that cap. A hydraulic jack is just a bottle jack turned sideways and when it pushes out it causes the other piece to move upward. You could watch some videos on how they work it was real interesting to me.

After watching 4 hours of videos and being satisfied I understood the general idea I was going to try to repair the old jack and then harbor freight did 40% off jacks that weekend and I realized spending a full day to repair a 30 year old Chinese jack my uncle bought for $30 before I was born wasn’t a good use of my time now I got a Daytona

 

[ajchien]

There are jack reservoirs with breathers and others that are sealed.

If yours was a rubber plug, it’s a sealed reservoir. you can find replacements on Amazon. Just search for “jack fill plug”. There will be a few different sizes. They are probably like a dozen for $10 or something like that.