It took some doing to open up the sender side. The plastic part is semi-transparent especially if I shine light onto it, so I can clearly see the o-ring through the plastic. It looked perfectly seated in its grove, nothing like what was shown in the pictures posted on-line. I was very baffled and decided to take the thing apart anyways. When I pulled the pieces apart, the o-ring just popped out of the grove! Turned out, it was broken. So even still seated nicely, it was no longer air tight.

Onto to ordering replacement o-ring and putting everything back.

Thanks to all the replies to my posted and guided me to this fix!