TM P226 Magazines

[Hero]

Hi all,

Got a couple of TM P226 magazines that are venting gas as soon as I'm trying to put it in. It's been a while since I've had to try and fix a leaky mag. I've tried taking it apart and retightening the screws holding the base plate in place.
Would I be correct in thinking that the O Rings in the fill valve have either dried out, cracked or need replacing?

If so - any place to easily get these from?

Thanks in advance! Google has not been overly helpful on this.