The "Y" fitting you are talking about is probably a tee put there as an
air vent and/or a place to pour bleach into the line to help keep it
clean. You are probably going to have to cut the line after that fitting
to clear it out. First, try and find out where it actually goes. A lot
of units here are tied into a bathroom sink drain. Look at the drains on
your b/r sinks and see if one has a pipe hooked into it from the side,
coming out of the wall. If so, that is the a/c drain. Sometimes they
will clog bad enough that you have to dismantle the drain there and
clean it out. I've run into them before like that. Also a couple of my
co-workers have blown lines out from the attic and went down into the
bathroom to find black goop all over the walls and in one case the
ceiling. I agree that is usually better to have the secondary pan drain
hooked up and draining outside where it is visible, and even better
still to have both a drain and a float switch. Anyhow, good luck and get
back if any more questions about the problem. Larry