I am assuming you are talking about a single-acting pneumatic linear actuator (air cylinder). If that's not the case please clarify. Single acting cylinders need a way for air to enter and exit the non-pressurized side, so an open port to atmosphere is required. Air flows in both directions. It is a not uncommon practice to place a filter on this open port to prevent potentially harmful debris from entering the cylinder. In most cases it would have nothing to do with condensation.

"Breather" plugs are normally designed to have minimal flow and simply allow the relevant annulus space to "breathe" and have no pressure difference due to temperature or changes in atmospheric pressure or very small fugitive emmissions.

you seem to be talking about something else "to allow air to escape in a high pressure cylinder?" but it is all very unclear. These are usually called vents.

Please provide more information (volumes, pressure, flows, diagram, picture etc) and a bit more explanation as to your question. I'm sure it's very clear in your mind and vision, but it's not in mine at the moment....

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I'm guessing you mean like a tom thumb or bimba pneumatic cylinder/actuator.

You can put things in the tapped hole that control motion and or noise, and keep the cylinder from inhaling and retaining crap.

https://www.amazon.com/Parker-047040004-Sintered-B...

Because it's a restrictive vent, it takes some of the slam out of pnuematic cylinder operation. The backpressure builds on the vented side as the piston nears it's end of travel. To some degree it slows to a stop instead of slamming to the end of travel, however you retain the full actuating force when the motion stops.

Of if you need to, in a pinch you could take out the return spring and make yourself a double acting cylinder.

Because a plug can be changed to a plug with a bigger or smaller hole if needed to tailor performance. Or can be changed to a sintered filter if bugs like to crawl into the hole or can be plugged....

I'm talking about a high pressure (2500 psi) hydraulic cylinder, single acting.

Don't any of the reasons given apply?

How about this:
Breather Plug - A vent installed into a port on a hydraulic cylinder to allow a double-acting cylinder to be used in a single-acting application. http://www.baileyhydraulics.com/hydraulic-terms

Breather vent plugs and exhaust port filters
For oil containers, hydraulic cylinders, and hydraulic systems
Hydraulic cylinders, which are intended to have a long service life must of course not be impaired in their function. For non-applied hydraulic cylinder spaces or oil tanks the danger exists that inflowing air may enable foreign bodies to enter the inside of hydraulic systems. These are indeed capable of causing major damage. http://www.haenchen-hydraulic.com/breather-vents-e...

Something else to take into consideration is seal failure on the piston.

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