I've just taken in a low-end but pleasant Eversharp Skyline, and discover that in addition to a rather leathery sac, there's no breather whatever in place. I've got some teflon tubing on hand, but I recall a recent thread in which... either Ron or Richard (memory and search strings don't provide) spoke some distain towards that material as a good breather replacement-- the context was Parker "51" repair, but I figure it's worth asking in this case too.

So, what's good?

That's what I usually end up using. The WD-40 tube is too thick and rigid, and keeps the lever from completely compressing the sac.

Where can you buy that tubing? Is it something one would find in a Home Depot?

Not at any Home Depot I've ever seen!
The tubing is listed online here.

The reservations concerning Teflon tubing largely concern its application as a replacement for the original sterling silver breather tubes as used on Parker 51 Aerometrics. Since silver tubing of the original dimensions isn't available, to my knowledge, the optimal available substitute would appear to be stainless steel hypodermic tubing. More than a bit of a pain to fabricate, however, for the casual repairer, for whom the Teflon offers an easily-worked substitute. The performance difference appears to be negligible, and has to do with degrees of leakage resistance during rapid and major changes in air pressure.

I've had no complaints about Teflon for the larger breather tubes such as are used on Skylines, Vacs, etc.

I've been satisfied using David's tubing for about a year and a half.
John

I make the stainless breather tubes, but it does take a jig to drill the hole, and a supply of the stainless tubing.

For everything but the the aerometrics that had the metal breather tube, David's tubing works fine.

...And that's why asking is better than working from memory. Thanks all, and apologies for any tremors I might have caused in David's tranquility.

Chalk it up to sleep deprivation. How's the young one doing?

We discover that Finn is actually an acronym:
Full
Is
Never
Near

So, he's doing well, but my wife's beginning to wind down some.

I replaced a Skyline breather tube once with an aerosol tube from the inside of eyeglass cleaner (finger pump type). It was small enough and slipped snugly into place. I had to make it shorter that the original tube to allow the lever fill to work properly.

An update: It's working brilliantly with its new teflon tube... or as brilliantly as it can, since a personal shortage of #16 sacs sees it currently sporting a pli-glass shorty which likely came from a Parker 21 and which I after some struggle freed from the sac protector of a Sheaffer Imperial. It's an inappropriate sac, but it's much less inappropriate where it is now.

I can see why people like these pens, too. Smooth, smooth, smooth, and I lucked into one with good flex.