A/C Air Handler Leaking Water Everywhere
The Tee was left open as a means of a clean out.

Start by using a shop-vac on that Tee and hold your finger over the other end, in the pump (you may have to get the pump out of the way).

Next, snake it out, going into the machine: (e.g., the brush that comes with an EZ-Trap; johnstonesupply.com)



Followed with another vacuuming. Ideally the first go sucks it out; you don't want to just push the clog inside. If it still leaks after all that, you may have a cracked pan.