Did IAD-YUL-BRU last November. Indeed you'll have to clear Canadian customs,
but there is a separate line for international connections (immediately left after coming down the escalator). It is a bit of a walk to get there, but it isn't too bad IMHO. It may take a couple of minutes for them to call up a customs agent; in my case it didn't take more than 5 minutes. The customs agent will open a door for you that immediately gets you into the part of the terminal with the international departure gates. You won't need to pick up your checked luggage at all. I'd say 1 hour should be more than enough.