Why do bottle jacks see differences in air pressure?

[Jupiter6]

I've noticed if you remove the filler plug on a typical single-stage bottle jack and pump it up and then push the rod back down you can hear air come out of the filler hole. It's a very slight pressure difference but it's there. You can see this by putting your thumb or a piece of scotch tape over the filler hole.

Is this a result of the air warming up inside the cylinder and creating a pressure slightly higher than atmospheric when you pump it?

 

[Mech_Engineer]

Bottle jacks are hydraulic- they operate through displacement of fluid volume. If you open the fill hole (and drain the hydraulic fluid inside) it will instead operate using air, although it won't be able to lift as much weight. When you push the rod back down you're just pushing the air back out.

 

[Jupiter6]

I'm doing this with all the fluid still in the jack. Jacks are filled with fluid to just under the filler hole.

In other words, if you take a brand new bottle jack that's been properly filled and pump it a few times, then remove the filler plug and push the piston down, you'll see and hear a small amount of air come out of the filler hole.

I suppose if you pump the jack, push the piston down and then remove the filler plug 20 minutes later you won't see any change in air pressure since everything has had a chance to cool down and come back into equilibrium.