For LC you can use two ports and use a second sterile syringe with polyfil as an equalizer. This is good for injecting the spores as well as taking out the LC. Using a different kind of filter in my experience doesn't work well because you end up getting the filter wet which can give way to contaminants entering. After all, the LC you want to shake up now and then to break the myc up and speed up growth.
There's no pressure build-up in the jar as far as I know - just that when you are sucking out liquid you are reducing the volume of the liquid and the air pressure is thus reduced inside the jar. The pressure you then feel you have to fight against is the difference between atmospheric pressure and the now lower pressure in the jar. Plus, chances are that if you closed the lid of the glass while it was warm - just after sterilization - you will already have low pressure in there, so if you insert a spore syringe without an equalizer it will suck the whole syringe dry... If there was pressure build-UP in the jar it would be easy to suck stuff out, no?
Silicone works well but I wouldn't reuse it more than twice. The PC overheats it a bit and it gets gooey after a while, at least that's my experience. Putting on a new drop after a round of nice growth isn't much of an effort to make sure... Also make sure the silicone is dry before you PC it, otherwise it will not work.
I really really liked the rubber ports you can buy though, they look and feel alot cleaner and more professional. But they are definitely not necessary.