Will Valium cover up marijuana in a pee test?

No.

When testing for illegal substances they have a few options, hair, blood, pee are the basics.

Marijuana will not be covered up by anything other that a lack of marijuana, 45 days to get clean if you are a regular 3-5 days for first couple of times.

But Valium will not cover it up, all that will do is add Valium to the list of substances found in your body