Possible faulty Schrader valve

In my experience, when a Schrader valve starts leaking suddenly, the most common reason is that there is a small piece of debris that prevents the valve from closing. The easiest way to remove them is to slightly overinflate the tire, then fully open the valve by pressing the center pin. If this does not help, the valve might be faulty and the tube should be replaced.

Check to make sure the valve is screwed in all the way. I put sealant in a tube for my son's bike, and the inner portion of the valve need to be torqued down. Fixed the problem. Also, it's not a bad idea to carry a valve tool with you on rides! Luckily I got to do this work at home in the garage. Would have been a real buzzkill out on the trail.