Can I ask a stupid question? What 'position' are you in when it escapes?
Standing, sitting, crouching down, laying down? Yes, it matters.
If it only happens in one position (crouching down for example) then it may mean that your pouch has 'slipped off of the wall' or 'twisted'. It also means that your pouch is fine, healthy but requires 'repositioning'. A much smaller surgery that can be done laparoscopically. They literally lift it up and reattach it to the wall (often using a mesh repair technique), test it and it's done!
In order to find out if this is the case, first, start noting what position you are in when it happens. (I found out when crouching beside someone in trouble, and suddenly 'whoosh'...scared me to death!) and then try filling the tub to the top and turning/twisting into different positions to see if there are bubbles that escape (Dr Cohen's technique).
If it is 'positional' then find a very understanding radiologist who is willing to 'play'.
Mine does an Opac/Contrast study. He injects the contrast liquid into the valve and takes pictures. Then he fills the pouch.
He takes pictures of my pouch laying flat, on either side, sitting and standing up then repeats the process after emptying out most of the contrast liquid.
That gives a much better image of what is going on in your body/pouch.