I have not heard much of the ink you are using. Googling the name led me to alotofthings.com, which is a reputable etailor. I have talked with the owner and purchased from him in the past. Maybe the lot of PGI-5 refill ink is incorrect and an email to the site might be helpful.
Is it possible the ink is diluted, contaminated or decomposing from age or environment?
Reasons for leaking ink and/or ink escaping via the vent hole could be:
1) Injecting too much too quickly. Ink injected into the cartridge must displace air within the cartridge. If the escape of air is blocked or curtailed then ink may also be forced out at the same time. The cartridge is a fixed volume and air and ink are only so compressible, so something's got to give when pressure inside the cartridge increases. There are only three routes available, barring a crack or other hole: The Durchstich hole, the ink exit port, and the atmospheric vent apparatus.
I have had air/ink bubble from the ink exit port while injecting, usually because of pushing too fast. If the ink exit port is not allowing the escape of air, then the atmospheric vent and Durchstich hole are the only avenues left for air/ink to vent.
2) There is an unknown hole or crack somewhere that is allowing air into the cartridge. This would be analogous to an improperly sealed top located fill/flush hole. Ink will leak out in this case.
3) Overfilling the cartridge. Ink will leak out until equilibrium is reached.
4) How many times have you refilled the problem cartridge? Could it be you need to flush the PGI-5 cartridge? Maybe the flow at the ink exit port or the sponge further on up are caked with ink and impeding normal equilibrium process while refilling.
Probably other reasons, but this is what I can come up with at the moment.
my thoughts are the air should escape from the point of least resistance which should be the exit port as the needle is snug in the hole that it goes through, if i cover the vent port while filling i force ink out of the exit port which says that the sponge has ink that the air must displace prior to the air getting out, so i am talking about 2 different problems, 1st the ink on the PGI-5 will drip slightly after being filled. 2nd filling this cartridge has a issue of ink being forced out the vent breather, so i have a couple plugs that lin showed from alotof things (the 1st longer plug that she shows) if you ream the canon hole with a 5/32 drill bit the plug will fit flush as possible so i may try this option
Hence out of all the above-mentioned seal plugs that I managed to try out, personally, I prefer the seal plug (e) as it fits well without any modification and has less hit or missed cases
Personally I use the class (a) design for years with zero problems as long as my hole is not large enough to allow easy plug movement. In fact my hole has to barely allow the plug to fit (so that I need to lubricate it a bit). This isn't a problem as the durchstich principle requires the plug to remain in its place.
I think the (c) design is the best. I do not think that the plug needs to be shaped according to the plug hole; that's why it's rubber.
Regarding type C plug.
SIZING
I will add some further comments about the shape of the plug and hole. In testing, it has become evident that while odd I have found that the hole size varies with different runs of cartridges and models. In other words, depending on the wear on the mould and mould itself, the hole size varies slightly. There is a significant difference in size between the PGI-5 and CLI-8. Thus making a rubber plug that will fit all cartridges perfectly is not possible. We will need to slightly modify the hole once and thereafter, the plugs will fit perfectly. There is no other way around this issue.
SEALING
Secondly, is the method of sealing. Lin had incorrectly assumed that the hole is sealed at the top with type C. This is not so at all. If you inspect the hole left by the Canon plug you will find that there is a significant constriction towards the bottom of the hole where it is distinctly narrower. This used to great advantage in properly sealing the hole. If you inspect the plug you will find that it is hollow most of the way until near the bottom. What seals the hole is that the solid part of the plug goes past the constriction and the constriction then seals the hole like a tightened belt by constricting around the hollow part of the plug. With this, there is a positive sealing always as long as the plug is FULLY inserted as it was meant to be. This is like using the constriction as a belt tightener.
I spent the better part of a year to research all the options and found type C to be the optimal and most reliable.
In my opinion, it is the worst to tamper the original seal and then try to re-seal it with whatever that has caused all sorts of leaking problems. If C plug is the best why suddenly there are different molds used by Canon to make the cartridges causing the hole to vary in sizes? If this is true then using a C plug or whatever sized plugs are no good solutions to me.
The best solution to this messy leaking problem is to use the German refill method, solution number one. Or to drill a new hole and leave the original seal alone, solution number two. The solution number two is actually the easiest to do. You can heat a metal pin by a candle or a match and pierce a hole through the plastic at the top of the reservoir tank like piercing a layer of wax. The hole can be as small as the needle tip. To reseal just use a hot glue or white glue (for wood work). The smaller the hole the easier it is to seal. It is so easy and I wonder why all the fuzz about tampering the original seal and struggle forever to make a perfect seal.
The only question you may have is where to drill or pierce this hole on the top of the cartridge. Just look for the front edge of the cartridge at the top. There is a round depression near the front edge (but before the LED light thingy). That round depression is you the best spot to drill and to seal. The plastic is about 1 mm thick there (or less). Just drill or pierce a hole there about 1 - 2 mm diameter, large enough for the needle. A drop of white glue will cover it up in a second and it will seal perfectly. You can peel it off in a second too next time you want to refill again.
It took me an hour or two to tamper with the original seal of an OEM cartridge once. And that's it and no more. A small tube of white glue at less than $2 will last for more than a year after many dozens of refill.