Tubes do not have pressure ratings -- the tires do. The tube should work at any pressure that the tire is rated at.

Neither presta nor schraeder valves need a certain amount of pressure to stop them from leaking either.

Dust covers just keep dust out -- they won't stop a leak in most cases.

If you didn't let the air out yourself accidentally, it is possible that there's a slow leak, though I know you said you checked. Tires do lose air over time -- high pressure skinny tires tend to need airing up every other day or so, but fatter lower pressure tires should last a week or two. But losing half the pressure overnight? Not normal.