downvoting doesn't drain upvoting, because they are in a separate pools now since the Hard Fork.
The downvote pool is of limited size though (2.5 full power downvotes per day). If someone wants to downvote more than that they have to use their regular upvote power.
Oh, so you can downvote after the (downvote) pool is empty, but from thereon it's taken from upvoting power, got it.
Yes that is how it is supposed to work and as far as I know does work.
Once your downvote power goes to zero, it is supposed to start using your regular vote power for downvoting. If you find that it doesn't, that's a bug, please carefully document the conditions and report it.