The Sheaffer Snorkel was a development of the pneumatic-filling Touchdown. Replacement of a Snorkel's barrel O-ring seal is the same as for a Touchdown (instructions here), noting only that Snorkels all use the smaller O-ring, common to later, slender Touchdowns, while PFMs (which are essentially oversized Snorkels) all use the larger O-rings, common to the thicker, first-year Touchdowns, and Touchdown Tuckaways.
Snorkels and PFMs have one seal that simple Touchdowns do not -- a point seal, which prevents leakage around the filler tube. If this seal leaks, the pen will not fill completely, and worse, may suck ink into the pen's interior, which can quickly result in destructive corrosion.
Replacement of the point seal is very simple on Snorkels, requiring only unscrewing of the nib assembly from the ribbed section. An example with the conical Triumph nib is shown above, but the construction is the same for Snorkels with conventional nibs. The new point seal is lightly coated with silicone grease, and the nib assembly is screwed back into the section, with no sealants or adhesives required.
Replacement of the point seal on a PFM is more difficult. The nib unit is disassembled by unscrewing a threaded coupler, which holds all the internal components in place. Once the coupler is out, replacement of the old point seal is easy (the parts to be taken out are shown above), but it's not always so easy to unscrew the coupler, which is cemented in place. Sometimes this cement has turned brittle over the years, and the coupler will unscrew with little effort. More often, however, heat is required, which is best applied by pouring very hot water over the back half of the nib assembly. PFM plastic is prone to distortion if only slightly overheated (which is easy to do with a heat gun), but will withstand near-boiling water without apparent ill effects. In some cases all that is needed is a bit of lighter fluid in the threads to soften the sealant (NOTE: use extreme caution in combining heat with lighter fluid!), which can be combined with gently heating the outer shell so as to reduce its brittleness, then squeezing it, turning it a bit, squeezing it again, and repeating all the way around, thus breaking the seal.
The other somewhat tricky aspect of Snorkel and PFM repair is sac replacement. The sac is attached in the usual way to a nipple on the back of a small, stiff rubber plug, which is tightly fitted into the both of a metal sac protector, which completely covers the sac. The plug also carries the filler tube, which is itself tightly press-fitted in place. It may be tempting to leave the old sac alone if it seems to be soft and resilient, but this is a very bad idea. Rubber that is 50 years old is not trustworthy, and when examined closely, you will find that old sacs are typically porous and on the verge of failure. And as noted above, ink leakage inside a Snorkel is no joke, since there are plenty of metal parts susceptible to corrosion.
Sheaffer used three different methods of crimping the sac protector's mouth to hold in the plug. Repair procedure varies depending on the crimp. The one shown at top is found only on very early production Snorkels, and consists of punched-in crimping directly over the side of the plug. The one shown above on the bottom is found on later-production Snorkels and PFMs, and consists of four sharp V-shaped indentations in the rim of the sac protector's mouth. The most common crimp on Snorkels, however, consists of an even inwards rounding of the sac protector's mouth.
The only type of crimp that requires any metal-bending is the later four-point indentation. Apply heat to the front of the sac protector, which will make the plug less stiff, and use a pointed tool (I use a burnisher) to pop the indentations out. You may wish to rest the side of the sac protector's mouth against a hard support, so that the indented metal doesn't get bent out farther than necessary, leading to fatigue and cracking. Once all four indentations are removed, heat the front of the sac protector once again, and use the Snorkel tube to wiggle the plug out. Pull, while moving the end in a small circle. The tube is easily bent, so be gentle! Padded pliers will give you a better grip on the tube, but also make it easier to bend or squash it. If the plug cannot be wiggled out, or if the filler tube pulls out of the plug, skip ahead to using a drive punch.
For the other two types of sac protector, go straight to the application of heat and wiggling out the plug by gripping the filler tube. This is usually very easy with the early side crimps, but less often successful with the sac protectors with the even inwards closure. For the latter, a much more aggressive use of heat is required -- a temperature high enough to soften the outer part of the plug before the inner part softens enough to release its grip on the filler tube. Often as not, the filler tube will be pulled out of the plug before the plug comes out.
In such cases, the plug must be driven out using a metal rod -- a drive punch -- inserted through the end hole of the sac protector. This technique should NEVER be used with the filler tube still in place, as it virtually guarantees breakage of the hard rubber "tail" of the long internal feed that extends through the filler tube.
Before you use the drive punch, however, check the state of the sac. If it is hardened, break a hole in the sac under the end hole of the sac protector. If it is still soft, use a pointed tool to hook it through the end hole, and pull it out. If this is not done, the metal rod will end up compacting the soft sac, jamming it into the front of the sac protector and leaving you with a real mess.
Insert the rod into the end of the sac protector so that its front end bears against the back of the plug. Heat the plug, and push against it with the rod until it comes out. It may take a good deal of heating, and the plug may come out a bit distorted. Not a problem, as it will pop back into shape if reheating while not under pressure. Watch that the plug doesn't come flying out and get lost, though.
Once the plug is out, it is time to make sure that there is no old sac residue left inside the sac protector. If there is, heat will soften it, but be careful in scraping the interior: the sac protector is thin metal, and if bent or distorted, will not properly engage with the threaded end of the filler plunger. To clear out petrified sacs, you can use a 7/32" drill, held and turned by hand (use a 1/4" drill for PFM sac protectors). Also clean off any remnants of the old sac attached to the nipple at the back of the plug.
The best method of cleaning out filler tubes is an ultrasonic. If the filler tube came out of the plug during disassembly, temporarily put it back in place so that a narrow-mouthed squeeze bulb can be applied to the back of the plug to force water through the tube to flush it out.
Trim your replacement sac so that it will fit inside the sac protector when the plug is reinstalled. Attach it to the plug nipple with shellac, and dust it lightly with talc. Snorkels take a size #14 sac, while PFMs use a #17½. Thin-walled sacs are NOT recommended (more on this topic here).