The values are actually save in the database with a separator. I think it is a pipe ('|') character.
So that is why the Field.Value == "1" will not work when more than one item is selected (the value will be something like '1|2', which is not equal to '1'.
The LIKE statement won't work because it is a SQL statement and not a macro/k# statement. Could you try Field.Value.Contains('1') as the statement?