Below is what the Zipfit website says about how much cork can be added:
“Adding more cork
Accessing the ports to add more cork to your ZipFits can feel like fishing at times, but if you use some needle nose pliers or a tweezer, we assure you it is very simple. Be careful not to pull hard on these ports, as you could rip them. When
adding more cork, ensure that you have both sides of the sleeve separated and that the plunger is sliding into the sleeve, not behind or in front of it. Start with just 1/4 or 1/2 of a tube to gauge how that feels and then add more as necessary. It is unlikely that you will add too much cork, but
if you have added more than 5 tubes to a single liner, be careful. Everything has a failure envelope and you could overfill the cork pouch. This is not covered under
warranty and is not the easiest fix.”