So progress.
Managed to use the X30 WITH the comfort insert and with only a tiny little air bubble. Might not sound like much, but it’s a huge step forward to me - I thought I’d wasted money on a useless product. Thanks to Thoughtful Gold for the tip to lock the valve.
So the process is as follows - fill the Bathmate up at the shower. Insert penis inside. Compress gently for the first pump standing up, and then flick the valve closed before allowing the gaiter to push back - this was where the air was leaking in earlier, since the comfort ring is too hard to form a proper seal while standing. This maintains some vacuum and enough of a seal to allow me to walk the few feet over to the throne where I do my sets.
Once I’m sitting down and the valve is pointing above horizontal, flick it open and then pump up to maximum pressure (I still need to use the Vacu-Vin mod to get to the vacuum level I’m comfortable with). And then rinse and repeat. I have to say that now that I’ve got it working, the comfort insert takes away the gaiter bite issue completely.