I can only speak about the pyrex tanks from Valley View Vapes. I got two of the 7ml pyrex tanks about a month ago. The base tank comes with huge silicone grommets, which take the place of normal end caps. These grommets are soft and pliable, but when assembled within a glass tube and a carto inserted, they feel firm and fit extremely tight. You aren't able to slide the carto out like normal to refill with liquid. Instead you use a blunt-tipped needle (sold on website).
You slide the needle either between the grommet and glass, or grommet and carto. Inserting the needle does pull a small part of the grommet into the tank with it, but a dental tool or o-ring tool can easily tease it back into place. I preferred inserting the needle in from the bottom side of the tank, so if you do tear the grommet, it wouldn't be so noticable. You could puncture it with a sharp needle directly in the middle of the grommet, but it would leave a puncture hole you could see, but it wouldn't leak juice. Kind of a drag filling, but you do get the hang of it after doing it once or twice.
The vendor has since begun selling black delrin and alluminum end caps. I got the delrin caps for my two tanks and I can now fill them in the usual carto tank way. These are very nice tanks for the price and the 7ml tanks offer unheard of juice capacity for a glass tank and look awesome on larger mods like a Silver Bullet. They are among my favorite tanks and receive my favorite juices. I have clear, and just ordered two more in the frosted glass. The vendor also just began selling the first glass tanks with a side juice fill screw called the Sidewinder.
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Triple V tank with grommets
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Triple V tank with delrin caps