Thanks Kien,
I heard the advice about the main pipe loud and clear, my not commenting on changing the main pipe was intentional
as I was hoping to not have to cut it since there is so little room in the overflow box to wield a saw and I didn't want to touch the bulkhead. I then noticed it was originally set up with a threaded fitting to the bulkhead in the overflow box so I could remove the main pipe completely without difficulty or then cut it once removed and reinstall if necessary.
BTW does anyone know of a good place to get a 2 " PVC gate valve in Calgary (preferably slip fitting?) I have been going to RONA and they have almost no selection on some of these fittings.
Palmer