I agree with Jonathon . It sounds more like a problem with the seal than the bag (although a cruddy bag could have a problem sealing). Try doubling up your seal. That’s what I do when I want to make dadgum sure that the seal holds. I seal like I always do, then move the bag just a hair and seal again so that the second one just overlaps with the first. That always holds.
As far as a brisket fitting in a Walton’s meat lug, it might or might not. A flat or a point would be no problem at all. A full brisket should work, but a large one might be a tight fit. I’ve seen some 18-20 pound monster briskets, lately, and I might be worried about keeping enough water in there! LOL
Actually, the larger issue with the meat lug is mounting the sous vide wand. The rim of the meat lug is too broad for just about any wand to mount with its own attached “holder thingy” (I’m not quite through my first coffee, so cut me some vocabulary slack LOL). It shouldn’t be much of a job to rig something up to hold the wand where it needs to be, but that’s something you’d want to deal with before go time.