Welcome to- The Fountain Pen Community!

A breather tube will allow ink to go back down the tube, but that's really not the function.

A breather tube is necessary for a filling system whose mechanism will not completely fill the reservoir with one cycle of compression and vacuum, such as a bulb filler.

There are three things going on -
1. Something to create compression and vacuum (bulb, piston, etc)
2. A breather tube.
3. A hole at the base of the reservoir.

When you create compression, air is squeezed out the breather tube, and also a little ink will go out the hole at the base of the reservoir. Less ink will go out the hole at the base of the reservoir, than air going out the breather tube.

When you create vacuum, ink will be drawn up through the breather tube, and spill into the base of the reservoir.  A little of ink will also come up the hole at the base, but it should be less ink than the breather tube.

The fact that the pen draws more ink through the breather tube than this hole, and also pushes out more air through the breather tube than the ink that goes out this hole is what fills the pen.  More ink coming up the breather tube and less ink going out the little hole causes a net increase in ink volume.

When writing, the hole at the base of the reservoir allows ink to get to the feed for writing.

Once the ink level starts to get as high as the breather tube, then the compression cycle will simply push ink back down the breather tube - you won't be pushing out air anymore. At this point, the pen is filled.

-- Edited by bgray on Wednesday 3rd of March 2010 07:02:30 AM

-- Edited by bgray on Wednesday 3rd of March 2010 07:06:24 AM