Water under your airhandler could mean one of several things.... plugged drain line.... dirty evaporator coil...or even low charge of refrigerant.
Make sure you have blown thru the 3/4" completely. You might even want to take it apart and make sure the "P" trap has no slime or as i like to call it..."Snot", which likes to build up in the trap. If all clear, then look up into the inside of the coil from the bottom if its an upflow airhandler (upflow units draw air in from the bottom, down flow pulls air in from above the coil)and see if the fins are dirty. As the units runs, it draws water from the air to the surface of the coil. The fins of the coil are designed to hold that water as gravity pulls the water into the pan but if the coil has a coating of dirt or slimey snot on them, then the water hits the dirt or slime, looses its grip and falls straight down.

Here is a picture of a extremely dirty coil i pulled out a couple years ago. It is turned on its back to see the inside. The pan is the black plastic part, which also needs to be clear of dirt, slime, even dead bugs.

If your coil is dirty, you should call an service tech and have them clean it. Remember.... if your coil is in this shape, no spray on cleaner you buy at a store will clean this completely. You need to have the coil cut out, washed and reinstalled. Finally, being low of refrigerant can cause a coil to ice up. Once its iced up, no air can pass thru the coil and it gets thicker with ice. When the home owner finally realizes this and shuts the a/c off, that ice will melt. Even if the coil is clean, the amount of water frozen in the ice is too much for the drain system to handle and it will leak. This too needs a a/c tech to be repaired properly.

Hope this helps

i used to design pellet stoves so i am up on typical construction. never had a cooling coil apart and assumed the drip pan was an open center design. i have good airflow at vents, vent temps are fine. i blew out hose and later came back and used shop vac to suck out plenum elbow for drain. got about 1/2 qt of water. maybe a bit more. unit did not run much last night. have fan on floor and no new water is leaking from unit but it is not constantly running so i am leery. wow, cutting out coil to clean it and than reinstall? how is plenum cut to remove coil? seems very labor intensive.

Definitely labor intensive and expensive.
Could be in the $500 range.

Depending on how old your system is you might want to consider replacing it. You could be having another expensive repair soon if its old.

What type of air filters do you use? Did you change them often or wait till they are filthy?

Cleaning in place can be done...but extreme care must be taken. There are several 'no-rinse' products designed for exactly this application.