James, I have had the same issue as you describe. Clearly it is related to the rapid increase in blood flow to the penis just before ejaculation, which makes your penis super hard and enlarged. This engorgement presses against the liner and your receiver must displace a little bit pf air to compensate. The end cap valve is suppose to release a little bit of air at this point to compensate, something you would not be aware of normally when everything is working properly. Usually when this problem starts occurring for me, it is a sign that the receiver and especially the end cap valve need a thorough cleaning. Run warm water through it, etc. Blow through end cap to make sure the ball inside is allowing full, unobstructed airflow to pass. You should hear it rattle when you shake it. Better to be proactive than to have your orgasm abruptly aborted just before climax after a long pleasurable build up. Truly a bummer. And has happened to me many times.
If this problem seems to happen even though your receiver is clear and functioning as it is suppose to, you might consider a notch larger constrictor or possible liner too.