Must be Parker 51 day. Is the aerometric filler a "friction" fit or does it screw/unscrew into the feed? Any removal tips would be appreciated. And, assuming the bladder has a leak, as I suspect, is the bladder replaceable, or is there a source for a replacement unit? Thanks.

I don't know about how it goes together, but you can find parts here.
Vintage Pens - Parts

The metal cover should come off. You can replace the sac with a latex one or a special plastic one. We have both types available. Good luck!
Giovanni

Tryphon Pen Repair Supplies

Soaking, pressure and success.....metal cover off, and very slight tear in bladder.......which would explain the slight spraying upon filling.....what size sac does the 51 take? 16 fits.........but might it be 18?

Just a tip, before you fit the new bladder, makes sure you get the nipple it fits on really clean. Look at it through a loupe to make sure. The one and only problem I had with an Aero "51" leaking was a breakdown in the seal at that point. I first just quickly stuck it back, but within a couple of months it was leaking again. Next time it was a new bladder and a complete clean first, been OK for about 6 months now.

Jim