You may not hear anything at all on the upstroke of the TD tube. Silence is not unusual. You should hear a "chiff" at the end of the downstroke.
A basic misunderstanding of how a snorkel works here....
The snorkel is a pneumatic filler. When you push the touchdown down (TD) tube, the air in the barrel is compressed, as is the sac inside the sac guard. There is a 1/4" long notch at the top of the TD tube. When the end of the notch slips past the 0-ring in the barrel, the air pressure is released, the sac expands, and ink flows into the sac. The upstroke therefore has nothing to do with the filling process. On the contrary, it produces a vacuum in the barrel, which is why it's especially important not to pull the TD tube back with the nib and/or snorkel tube immersed in ink. The vacuum created may draw ink in past the point holder gasket around the snorkel tube and into the barrel instead of the sac. Note that the sac itself can't expand because it's encased in the sac guard. To fill, put the snorkel tube in the ink, push the TD tube down, wait 10 seconds, withdraw. If you want to do it again, don't immerse again until the TD tube is all of the way back. Not that there is much benefit to that, but some folks like to do it a couple of times.
If you have been cycling the TD tube up and down to pump ink into the pen I strongly suggest that you take the pen apart and check to see if it's wet inside. If it is, dry everything off and don't think of using the pen until it is serviced.
This is incorrect. For the downstroke to produce maximum compression, thus deflating the sac, the upstroke must not produce a vacuum in the interior volume when the mechanism is at full extension. Therefore, the vacuum that is created while the filler is withdrawn and the volume in the chamber increases must be released at the top of the upstroke so the pressure is equalized with that of the atmosphere.
There is, of course, a short notch or groove at the top (knob end) of the Touchdown tube that breaches the seal between the tube and the O-ring when the plunger is depressed, allowing the pressure inside the pen to be released and permitting the sac to expand and resume its natural shape, drawing in ink in the process. However, there is also a vent hole at the bottom (nib end) of the Touchdown tube (a simple hole in Snorkels, a rather more involved depression in a non-Snorkel Touchdown pen). This vent hole releases the vacuum at the terminal extent of the upstroke of the filler. Here's what the vent hole looks like in a Snorkel:
This feature (labeled feature 61) is clearly described in the Touchdown patent:
"One of more longitudinally extending grooves 60 are also provided on the forward end of the tubular member 40 so that when the tubular member is in the fully extended position the groove 60 spans the packing gland [O-ring] 54, thus again venting the chamber to the atmosphere. Thus any vacuum created when the tubular member 40 is extended is dispelled through the groove 60 prior to the time that the compression stroke is commenced as above described. In addition, tubular member 40 is provided with one or more vent holes 61 in the groove or grooves, these vent holes also being positioned so that they are positioned to the read of the packing gland 54 when the tubular member is fully extended. If desired, the grooves 60 may be omitted, the function thereof being supplied by the vent holes 61. In such a case any vacuum created in the chamber will, upon full extension of the tubular member, be dispelled through the vent holes 61 and the annular passageway between the tubular member 41 and the sheath 30."
Indeed, when the filler is extended on a properly-functioning Touchdown, there is an ingress of air at the top of the stroke; this can be clearly heard, though likely by virtue of the air being draw in, rather than expelled, the sound is not as obvious. If there is no such sound, or, as has been described in this thread, if there "is a vacuum being made within the barrel which pulls the plunger back down with it," it is an indication that the vacuum is not being released, and the filler may not function properly, as the sac may not be sufficiently evacuated on the downstroke.
I suggest examining the vent hole in the pen under discussion. My suspicion is that you will find that it is blocked, perhaps by some debris from an old sac, or by a bit of shellac, or corrosion, or random crud.
Edited by kirchh, 02 September 2014 - 18:01.