new tube..still leaking

[Dogmama]

I had the same problem & turned out my rim tape was worn. I couldn't feel anything sharp, but apparently one of the nipples was wearing a hole through each tube. Very maddening, as I changed out tubes repeatedly & finally put a new tire on - same thing. I sheepishly took it to my LBS & they replaced the rim tape with something really cushy. No more flats (at least not from rim tape failure.) Shoulda figured - the rim tape was about 10 years old & had about 60,000 miles on it.

[Running Mommy]

Thanks Dogmama- I was about to pipe in RIM STRIPS... But you beat me to it.
Check yer rim strips ladies!
Some rim strips are better than others. I prefer the ritchey ones that look like bright yellow strapping tape, but velox is good too. Cheap rubber- not so good. It tends to shift.

In the shop we always check not only the tires but the rim strips. Also when changing a flat, take the old tube and find the hole/puncture. Then lace the tube around to the area it was at when it blew. That will help to find the problem. If it's on the inside of the tube chances are it's a rim strip problem. If not, look for something imbedded in the tire in the area. Or if it's a snake bite, well... It's just a snake bite. Either you took a hit and it pinched, or you didn't pump (I like to say "puff", but my mechanic mocks me ) up your tires before the ride.
And then there are the classic tube failures of a defective tube. Those are generally along the seams, and tend to be pretty big. I really don't find defective valves all that often. But sometimes the shop guys will tell you thats what it is just to get you on your way. It's just easier for them than really trying to figure it out.
Yeah, I know....
Anyway,
ALWAYS check your tires before every ride. And when I say check, I mean WITH A GUAGE. Road tires always seem "hard enough" even when they only have 85-90 psi in them.

[boy in a kilt]

This is a cheap trick, but I always align my tires so that a label is aligned with my valve stem. That way, I know where to look for the hole in my tube or the piece of glass/rock/nail in my tire.

[OakLeaf]

okay then for you mechanic types - what's a good gauge for Presta valve tires? I've always just used the gauge on the pump.

I know not to trust anything but BluePoint (Snap-On) on my motorcycle tires - and to get that gauge calibrated by the race tire guys every few years. What level of accuracy do I need on my bicycle tire gauge, and where do I find it? (And how much air do you typically lose checking high-pressure tires, if your technique is very good which mine isn't always ?)

ETA: another thing I've heard from several people in the last year is that 90 lbs is plenty for a lighter rider even if the tires are rated for more. Touch wood, I've actually never in my life had a pinch flat even when I was 150 lbs. Does the rider's weight affect the minimum tire pressure to prevent pinch flats?