Originally Posted by
willydstyle
I had a leaky presta valve core, so I removed the core, put some grease on the threads, reinserted the core and then wrapped the core/valve stem junction with a single layer of Duck Tape. It's working great, no more slow leak!
Try using plumber's teflon tape. It should work better even. But that begs the question, why not just replace the core. They don't cost much at all. I bought a box of 10 for $10, including the tool.