Originally Posted by
JenCor
Thanks for all the help. I've patched and slimed the old tubes and will be playing with the presta one outside of the tire but so far can't get it to behave like it did.
You might fill the tube and then hold the valve area in a sink of water to see whether you spot any air leaking. Wiggle the valve around to help expose any flaws.
You mentioned tightening the little nut, which makes me wonder if in doing so you tore the valve somehow.
Possibly you just have a defective valve.
Have you gone through the 15-minutes-then-lose-pressure cycle more than once?