Are you absolutely sure it wasn't your kid letting the air out? Actually, Schrader valves with caps don't leak even if there is debris in the internals. As long as the seal with the rim & the exterior of the valve is intact. Did you change out the valves when you changed the tires? If not, next time, put on some Bridgeport's. My Honda shop manual says to replace the valve stems every time you replace the tires. This seems excessive & I don't know anybody who does this, but that's what they say.