I'm guessing these came with Zipp's balck plastic valve extenders, right? Those things are just not very good. You can't get a good pressure reading when inflating. Also, the valve head, being left open, is prone lose air.

If you don't insist on latex tubes, the best option is to never use a valve extender at all. Bontrager, for one, makes an 80mm tube that works fine with Zipp 404s, which I believe are the same depth as your flashpoint 60s.

If you are determined to go the valve extender route, here are a couple options I have tried that work OK:

1. TNI extenders for removable core tubes. The hitch here is that you have to have a removable core on your inner tube. (Conti and some others makes them). If you go to a bike shop, there is roughly a 25% chance that the clerk there will know what a removable valve core is--most do not and I have had a few try to sell me tubes that had fixed cores. So buy your tubes online. To use this extender, you screw out the valve core from your tube (use needle nose pliers for this), screw on the extender, seal that junction with teflon tape, and then screw the valve core into the end of the extender. You can find them here: http://www.tri-zone.com/Details.html?cat=95&item=TNEXT

Buy 3 so you can have your spare tube prepped with the extender installed in case of a flat.

TUFO valve extenders appear to work the same way.

2. Topeak makes valve extenders with an internal piece that you can use to tighten and loosen your presta valve head. The beauty of these is that you can use 1 extender for both wheels and actually shut the valve head when you are done inflating and then remove the extender. This requires a little "feel" to do properly, but the end result is a pair of invisible closed valve heads inside the wheel rims and a spare valve extender that you carry in your blowout bag.

With both of these options, it can sometimes be a little tricky to get an accurate reading on your air pressure gauge, but it is possible. Learn what your ideal pressure feels like when you press your tire. Don't depend on that skill, just use it as a reality check when inflating.

With my one experience renting 808's for a race, I had trouble with one tire of the two not wanting to be inflated. It seemed like the valve extender was leaking at first, but I realized it was actually my pump head that was not sealed tightly. It actually responded to forceful pumping, but required some manhandling to keep the pump head tighty sealed around the extender. I discovered CO2 also worked because of the extreme force of inflation.

I made an L shaped pin out of wire for my blowout bag. I used this to tap the valve head down inside the valve extender. This allowed me to deflate the tube completely or just tap/release the valve from sticking. This was especially useful when my CO2 froze it in the open position.

Luckily for me, I got them set and never had to change a flat during the race. And then, I just sent them back. But it was a good lesson for me in being prepared for race day.

I hope something in there is helpful for you!