Yes, Aerometric Parker 51s can have hidden problems, too

It's often said that the Aerometric (squeeze-filling) Parker 51s of the late '40s to early '60s are virtually bulletproof: that even as-found examples usually need no more than to be cleaned out by repeated filling and emptying with water, and that their Pli-Glass sacs are nearly indestructible. There is some underlying truth to all this, but at the same time it obscures the fact that Aerometric 51s do have their vulnerabilities -- and that there are good reasons to have one properly serviced for best results. In fact, the ease with which most long-neglected Aerometric 51s can be put back into working order can be a bit of a trap, for many who offer Aerometric 51s as reconditioned (or who offer reconditioning services) don't bother taking the extra steps to make certain that they are truly 100% restored.

Proper reconditioning will entail complete disassembly. Removal of the hood (shell) is essential, inasmuch as the collector (at the left above) is prone to clogging -- and when dried ink residue packs the finely-ribbed ink trap, it often cannot be removed by soaking and ultrasonic cleaning alone. A 51 with gunk in the collector may work, but it will be more vulnerable to flooding and ink flow may not be consistent.

A 51 that isn't completely disassembled may also have problems with its breather tube. Again, the pen may still function, but less than optimally. The original sterling silver tube shown above has corroded away in patches, leaving holes in its side. New tubes in stainless steel are not expensive so there is really no excuse for not replacing a damaged original.

Surely the most commonly neglected Aerometric reconditioning step, however, is making sure the connector is sound. This is the component that holds everything together: hood, clutch ring, nib assembly, sac, sac housing, and barrel -- all are mounted on the connector. For the first few years of Aerometric production the connector was made of machined acrylic, with threads at the back to attach the metal sac housing. Thereafter connectors were made of injection-molded styrene, with the metal sac protector redesigned to be mounted with a firm press fit. The earlier acrylic connectors (bottom, above) are pretty much invulnerable, though they sometimes will break when too much force is applied in removing a sac protector with corrosion on its interior threads. The styrene connectors (top, above) are another story, as they are notoriously vulnerable to plasticizer migration from the PVC of the Pli-Glass sacs. The usual result is softening of the sac nipple, but the connectors below show even more dramatic damage. These are Parker 21 connectors made from the same material, new old stock stored in a bin with sacs attached. The plasticizer from the sacs not only turned the material soft and rubbery where they were attached, but also melted it where the sacs rested against the sides of the connectors during years of storage. 

Damage to the sides of a connector is not going to happen under normal circumstances, yet sac nipples that are now the consistency of Silly Putty are all too common. Knowing this, many repairers will just turn a blind eye and simply not take off the sac protector so as to avoid the risk of having to deal with the added hassle of either replacing or rebuilding the connector. For repairers who are doing it right, however, cutting off the softened nipple and replacing it with fresh material is just part of the job, and one that doesn't take long with a lathe and the necessary fixtures (a repaired connector is shown below).

Finally, it should be noted that while Pli-Glass sacs may remain functional for decades, they do stain rather easily and once they start to go, can release some rather nasty acidic deterioration byproducts. Since new reproduction sacs are available, original sacs that are not in great condition really should be replaced even if they are not visibly leaking.

UPDATE: We've made a video illustrating these points which can be viewed on YouTube here.