Review on Presta Valve Core Pack (12 Units) + 12 Anodized Alloy Presta Valve Caps for Tubeless Road & MTB Bikes, Compatible with Stan's, Vittoria, Continental, Kenda & MORE. LeaK-Proof Replacement for Presta and Schrader Valve Cores by Matt Conner

I fitted these to my mountain bike and the tire went flat within a few hours. I replaced the core with another one from the pack and it held up. The thought was an accident. Well after a few days I installed them on my fatbike - same problem. Basically you have to keep installing them until you find one that will hold air. Doesn't inspire much confidence if you're driving in the woods and don't want to deal with a flat.