KWA gas fill valve seals

[Mad dog 49]

Am I being a numpty here or what? 

I've got KWA M4 gas mags and for some unknown reason they seem to shit out the tiny little fill valve external seal when I fill them. I've got 4 mags, every one of them has worked great out the box for one game day and then by the end of the day, they won't take more than a small bit of gas without spraying gas out the bottom. On inspection, the tiny little donut o ring fill valve seal is missing. 

I fill the mag the same way as everyone else, upside down, short fill and let it settle, bit more fill, and so on until its full so, am I supposed to replace these pesky little buggers after every game day or are they supposed to be sealed in somehow and on mine they aren't?

I've been running them recently with HPA taps because of their lack of ability to hold gas otherwise and because the weather is cold, but I'd like to go back to gas because, well, hpa is the devil ?

 

[Mad dog 49]

I've changed out all the fill valve seals this morning. Used LPE silent fill replacement seals and they are not leaking or showing signs of falling out. One of the originals I removed had a small tear in it, so perhaps the standard seals are a bit poo and need changing out, or perhaps they've been sat in a store somewhere for ages and have dried out too much for a bit of silicone to fix. They do seems quite a bit narrower than the LPE ones so all being well, it's all good now.