Just a curious thought-what percentage of your quad use is work (yard, field, hauling, etc) vs playing (exploration, recreation, enjoyment)
personally mine is about 60/40 work to play. Most of the work is winter plowing!
In my case, it was the o-ring that goes on the seat for the needle valve. I saw that you replaced the o-rings, but did you check this one as well? Mine looked fine, but was just a little too loose. Installed a new one and vent leaks all gone