Dim Sum after 2pm on a Sunday near Silver Spring?
If a 20 minute drive would be considered close enough, A&J in Rockville serves Taiwanese/northern Chinese small plates from a menu all day. Not Hong-Kong style dim sum, but reasonably similar, and excellent.
Hollywood East serves only a small array of their dim sum after 3pm from the menu, obviously not from the carts. Oriental East does not serve at all after 3pm. And both stop cooking before then, so I wouldn't trust the freshness of anything served in the last hour.