First stage issue

[GJC]

That must create a lot of wear and tear on the o ring. No wonder why they need to be changed frequently. I'll continue to stick with DIN as much as I can.

 

[uncfnp]

If you look carefully at the oring seat in the tank valve, you may see a little eccentric lip. This feature enables a little air to slip behind the oring to speed its seating against the land of the first stage.
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A brief hiss/pop that you hear is the escape of a tiny amount of air before the oring seals. It is completely normal.

This feature is common to "thin o-ring" tank valves. The thicker o-ring versions don't always have this.

Having said all this, your sound on your second video is a bit more than I would expect. The first video sounds like the usual sealing pop. But I've had a couple tanks that always seemed problematic in sealing, and would hiss endlessly after a vigorous opening. I assumed it was my choice of o-ring as I substituted them over the years. If your yoke bolt is tight (no swivel possible when pressurized), and if you seal completely after the initial hiss, I wouldn't sweat it.
Why? Because if the yoke bolt is tight, you've eliminated the problems:
1) you localized it to the tank/1st interface.
2) you were able to obtain a sealed condition
3) your IP is stable, eliminating other possible causes

You can prove this to yourself by trying several tanks. Each one will be different, due to tiny differences in the seat design. And not all may seal. I've had two thin o-ring tanks over the years that I grew to really dislike for a very similar problem. I bought new valves for them for the next time I emptied them. $50. Problem solved. In your case, it may be even easier. If lubing the o-ring doesn't help (by making it easier for it to shift in its land), then just try a new DIN insert.

Read this too late but the stages really did need servicing anyway. And I had forgotten that one of the seconds is the one that spiraled to the ocean floor on my last trip. It was in rough shape. Got them all done but you’er right, not much change in the swish. As I said, have never really used these tanks with the inserts before and was started by the sound and air escaping.

I still need to tune the stages then I’ll try a few more tanks tomorrow. Maybe another reg set.