Valve cores

[saddlesore]

  While checking the tire pressure  I unscrewed the cap and had a bit of water and air bubbles.  The tire pressure was OK so I thought the rubber gasket in the cap was the only thing holding the air in.
  I replaces the core but still had a leak.  Tightening it didn't stop it so I backed the core out and got part way and the leak stopped.
  Does  the air pressure help seal the valve core?  No more air bubbles.  I don't want to depend on the valve cap to hold the air in.  Or am I riding on borrowed time?   

[hubcapsc]

  While checking the tire pressure  I unscrewed the cap and had a bit of water and air bubbles.  The tire pressure was OK so I thought the rubber gasket in the cap was the only thing holding the air in.
  I replaces the core but still had a leak.  Tightening it didn't stop it so I backed the core out and got part way and the leak stopped.
  Does  the air pressure help seal the valve core?  No more air bubbles.  I don't want to depend on the valve cap to hold the air in.  Or am I riding on borrowed time?   

If you still have the OEM valve stem, you're riding on borrowed time.
It might be that you can get the leak started again, well enough to
hear, by wiggling your OEM valve stem...

-Mike

[Gryphon Rider]

It sounds like the rubber part of the core that seals against the inside of the valve stem (the red part in the image below) is failing.  I would replace the valve core, were I you.  If you are using the original valve stem, I would certainly replace that with an all-metal one at the next tire change.

EDIT:  I re-read your post and saw you replaced the core.  Perhaps the surface inside the stem that the rubber seals against has a nick in it, or perhaps some other contaminant?

[old2soon]

I realize tightening procedures are a matter of personal feel. How long has the valve stem been on? I M H O a valve core should be lightly snug. You SHOULD be using a metal valve cap-NOT plastic or rubber. IF you are the LEAST bit skeptical of the valve core or stem-replace with a new one. For a couple bucks better safe than sorry. Do you have dyna beads in your tires? If you do your supposed to put a squirt of air in BEFOE you check the pressure-in NOT out. Hope this helps!   RIDE SAFE.

[KY,Dave (AKA Misunderstood)]

Do you use a type of balancing beads? These can cause problems if not ensured they stay put after adding air.

[Firefighter]

Like everyone says, you need metal stems first. If you already have those, might try taking the core out and blow high pressure air into the valve stem so as to clean out anything that might cause the leak. Also, your new core could be bad.

[saddlesore]

  I tried a couple of valve cores.  I have the metal valve stems with metal caps and the caps have a rubber gasket inside of it.  I have ride on in the tires. I thought that it's what gunked up the first core.
  I always preferred the long cores but haven't found them in a while.
  I also have a tool to clean the threads on the inside of the stem.

[hubcapsc]

Sounds like you have all good stuff... I got new valve stems
in the Mini Cooper last time I got tires... first time I checked
the pressures, one (maybe two) valve cores got stuck and
air just whooshed out until I pressed the tester down
on them again. I finally got them to work right and
the tires aired back up... just cheap crummy new (short)
valve stems I guess...

-Mike