I assumed the Vacumatic diaphragm should be cut so that the plunger does not quite stretch it when pushed down but Richard Binder's reference to "the magically correct lenghth" tells me that perhaps I should pick brains better than mine on this subject.
Any opinions from experienced folks?
Thank you,
d

I don't see the reference to a magically correct length. He does show a cutaway illustration of how long it should be. He says that when the plunger is extended it shouldn't stretch the diaphragm, but there shouldn't really be excess rubber.

I've used this method with great success.

Hi Elaine,
Thank you, I guess my guess was right, that's reassuring. I was referring to a Feb 14 post by Richard:
"A Vac will fill only as far as one pump stroke's worth above the top of the breather tube. Is this pen's breather tube abnormally short?

The "popping" is a diaphragm that's not quite cut to the magically perfect length. As long as it doesn't jam, it's not a problem.

Richard Binder • Fountain Pens"

There was an article in the one of the recent Pen World magazines that the author stated he cuts all of his vac diaphragms to 1.125". I have tried that with the last few (4 or 5) vacs I have fixed and it is a good starting point. That said I have not done any OS vacs, so that could be a different ball of wax.

I trim the diaphragm, invert it after coating it with talc, press the pellet into the filler socket and then slowly work the edge of the diaphragm up onto the retaining collar until it butts up against the step. Once it is there, I cycle the filler and adjust the length shorter or longer until it looks good. What you have to remember is that once you push the edge of the diaphragm past the step on the retainig collar, it will reduce the stroke of the filler slightly until trimmed.

If you push the edge too far past the step on the collar and then decide to trim it when the diaphragm is nearly everted (by pushing the filler), if there is any diaphragm caught between the retaining collar and the threaded collar, then the diaphragm will be too short. Horribly long runon sentance, sorry.

It's kindof a feel/experience thing. I have only trimmed 1 diaphragm too short, and that was because I misred my ruler :doh: . On the other hand, too long and you will bind up on the barrel interior wall, or hit the breather tube, both big no no's.

Hope this helps.

Cheers,
SMG