With most pens, you can tell pretty quickly if they've been repaired correctly.  OK, sometimes you’ll open up a pen someone else has worked on, and there will be a cheap, one-piece "J" spring instead of a proper linked pressure bar, or the sac will be the wrong size, or the old sac won't have been cleaned out properly.  And too often the nib and feed haven’t been cleaned out or checked for function.  Still, it's when you come to Vacumatics and Vac-filling 51s that you have to brace yourself.  Nearly always, we end up having to redo them, even if already "repaired".  For while Vacumatic fillers may seem simple to fix, most repairmen stumble over a few basic details that make all the difference.  Here are the seven key failings:

• Failure to clean out old rubber and sealant residue. Hardened rubber often bonds to the diaphragm seat and becomes nearly indistinguishable from it.  If this residue is not removed, the filler unit cannot seat properly.  Rubber remnants sticking to the inside of the barrel will also interfere with proper operation, and sealant left on the threads of the barrel and filler unit can make reassembly difficult and awkward.
• Overtightening the filler unit on reassembly. If all components are properly cleaned first, reassembly should not take much force at all.  The diaphragm should be sealed to its seat by a thin coat of shellac and the lightest of pressure; torquing down the filler is unnecessary, and puts excessive radial stress on the seat area that will cause bulging on Vacs (this may become apparent only days or weeks later) and peripheral cracking on 51s (whose material is less elastic, and so more likely to crack than to bulge).  This may not be the method advocated in Parker's original service manuals, but it is the best method as borne out by the passage of time.
• Scoring the inside of the barrel. Rubber residue sticking to the inside of a transparent barrel can be difficult to dislodge, but if one uses a screwdriver or something similar as a scraper, the scars will be deep and clearly visible. 
• Failure to trim and lubricate the diaphragm. Too-long diaphragms will hit the breather tube and are more prone to getting folded up upon themselves.  Diaphragms without lubrication on their interior surfaces will soon bind and tangle.  Note that we strongly endorse the use of talc rather than silicone grease or oil.  Experience has shown that only dry lubes have sufficient lasting power and preservative properties.
• Using too large a sac.  The smallest size of Vac diaphragm is the only one you ever need use.  Although this is generally called the "Debutante" size, it is the only size that can be used in 51s and standard Vacs without binding on the barrel walls.  This size can also be used on oversize and Maxima Vacs, although the "standard" diaphragm is a better fit.
• Failure to align the blind cap on reassembly. This is a bit of an art, and it basically has nothing to do with scribing lines and replacing the filler unit in the exact original position.  A skilled repairman should be able to get the fit right every time (sorry, this is one trade secret we aren't going to disclose).
• Failure to clean out ink residue on Vacs. Usually a great deal of the original barrel transparency can be restored by a thorough cleaning of its interior; too often this is neglected.