Once it is empty it should just unscrew.
Some of the early air tubes had a valve in them, just like in a bicycle tube, later ones didn't.
You'll find that the reg has a brass nose on it, with a small O ring, I think there is sometimes another bigger O ring where the tube meets the reg body but my memory is a little hazy here, as it's a few years since I've had a Pro-T apart.
Anyway, whatever is there, replace with the best quality O rings you can get. I used to buy mine from a local hydraulic supplier. I told them I was working with air at 300 bar and they told me what would work best.
"I'm your huckleberry, that's just my game"