I know from my own experience with a suprapubic, which I had for about six months, that the leg bags didn’t have nearly as much vacuum as the night bags, but I’m not sure exactly why. It could be because of the longer hose, or maybe the valve, I’m not sure. The only time my leg bags would drain decent was when I was in a standing position.
I also found that it was crucial not to allow the hose to get a loop in it, by which I mean at no time could the urine be expected to go uphill, not even a little, except right at the catheter itself. When I drained my bag I also wouldn’t drain it completed, as sometimes that would cause it to lose some vacuum as well.
You might also try the CareCure forums at Rutgers. Just Google it. It’s a great forum for people with SCI.