How can I tell when the septum on my Mini GC needs to be replaced?

Signs that the septum on a Mini GC is going bad:
1) The injection feels too easy. With a good septum, when you insert the needle into the injection port it should feel like you're poking a pin through a sponge. There should be some small resistance. If you know you've done over 150 injections, it is a good idea to replace the septum anyway because it is likely to need to be replaced soon.
2) The Mini GC has trouble maintaining a high pressure (above 10 kPa) or reaches a desired pressure very, very slowly. If the septum is is really bad shape, you can even feel air leaking if you put your finger over the injection port.

To replace the septum, use a coin (nickel or quarter) to unscrew the injection port cap. Remove the old septum and drop in a new one, making sure the septum is sitting upright over the port.