I've only scanned the responses but:

(1) it certainly does not take more than 20 psi to "seal" a presta valve. A presta valve will totally hold air in a tube that is slightly pumped up outside of a tire, and that's less than 20 PSI.

(2) make sure the valve core is tight into the valve stem. Some (most?) presta valves cores can be tightened and loosened and removed. Make sure it's tight. I'm talking about the valve core, not the valve closing mechanism. I've had many a tube with slow leaks solved by tightening the valve core. Hope that makes sense. In the attached picture you can see little flat areas that you'd use to loosen and tighten the valve core. They also show a special tool, but a needle nose pliers would work fine.




(3) tube outside of the tire - submerse it bit by bit, slowly, in a sink of water. Even at the low pressures needed to expand an innertube outside of a tire, it will show bubbles at the leak. Sometimes you you have to look very carefully and move the tube around its circumference very slowly. It certainly doesn't take more than 20 PSI for the valve to hold air and to show bubbles. I'd start at the valve since it might be the problem area. If so, see #1 above.