The other main use of the latex sac is the button filler. Parker developed the idea from previous patents, most notably that of John T. Davison (1905). Parker used that design almost exclusively for several years. The idea caught on in the UK; Summit and Mentmore produced excellent examples of the button filler. Conway Stewart produced an elegant version with a fixed pressure bar which made sac replacement simpler and straightforward. They used it on various Duros as well as the 226 and 485.
The button filler works very well if it is set up properly. The very short travel of the button is enough to flatten the sac completely and allows for a full draught of ink. Problems can arise if a replacement pressure bar is not cut to the exact size required. Sometimes people have trouble removing the button. Mentmore recognised that difficulty and made removal and replacement easier using their threaded button.
The first version of the button filler flexed the pressure bar between the button and the section which had to be threaded to resist the force. Later Parker Duofolds used an anchor bar which moved the pressure of depressing the button from the section to the end of the barrel enabling the use of a friction fit section, a saving in the manufacturing cost.
Novices get into trouble by unscrewing the section first, twisting the pressure bar against an old, possibly hard sac. The proper way to disassemble a button filler is to remove the button and slide the section out, then unscrew the section to give access to the old sac.
Parker’s last version of the button filler was the elegant solution known as the AF or aluminium filler produced at Newhaven. Though the principle is the same the aluminium button and housing gave a modern appearance.
One of the benefits of the button filler (apart from ability to shoot ink at your classmates) is that the barrel is not pierced as with the lever filler and a pattern is unbroken from the section to the blind cap. Some pens such as the Mentmore Autoflow continue the pattern into the blind cap itself.
There was always the possibility of losing the blind cap. Mont Blanc and Stephens solved this problem in different ways, both utilising a fixed blind cap. Is the button filler a better filling system than the lever type? It’s a matter of personal preference, I suppose. I find it a more efficient engineering solution and I prefer it.