It shows up to 1 atmosphere of vacuum maximum because its impossible to create more that one atmosphere of vacuum, or -14.7 psi in gage pressure. Vacuum is really just a creative way of describing a situation where there's a lack of atmospheric pressure. Its like when you run your vacuum at home, the motor and impeller create a lower pressure, and the atmosphere around the nozzle forces itself up the hose along with the dirt... I haven't looked at the actual fitting specification in a long time, but in most cases these fittings are meant for air, in which case a small leak would only slightly change the level of vacuum or the flow rate. In fuel, the additional air and its effect would be quite obvious...

All that Harley and I are saying is that the barb fitting is far superior, which is an answer to the original question. Regardless of the spec I've seen union fittings leak under both circumstances, but I'd bet on them being more likely to fail under a vacuum. Additionally, a low restriction fuel system will lessen the likelihood of this problem, in that it will keep your fuel system closer to atmospheric pressure at high flow rates... This is done through the use the largest hoses possible, shortest lengths, etc. A side effect of that will be lower pump voltages, ECU potentially longer battery / pump life, probably lower EMI from reduced current, maybe even a more accurate RPM definition at idle and wide open...

Kelly