It's funny you should blame the O-Rings not being "captive" because I have plenty of 3/8" I.D. tube barbs here from DD to do a loop that I use for my testing and not a single one has ever leaked. I've used them on several different BI's, an Alphacool top, Csp-Mags, and I've milled the captive grooves off all my old Apogee's. If you know what your doing, you won't have problems. The trick to sealing O-Rings properly is to tighten them with your fingers until the O-Ring provides very light resistance, then use a wrench and add 1/2 turn at most. Another trick is to not set your loop up where the tubing puts a torsional twist on the fittings. Do those 2 things, and a O-Ring will never leak on you.