It seems this started when the outside unit was replaced after having failed at about 13 years old. I believe it's a 5 ton unit. Shortly after that the overflow pan on the air handler started filling with water.
The drain line was cleaned twice, then the drain line was adjusted to make sure it had a good slope, then it was replaced with a different kind of PVC and added a trap which it did not have previously.
None of that worked so then a pump was added, but the overflow pan still fills.
We were first told that there might be damaged to the main pan, then were told that the outside unit is incompatible with the air handler, something about the expansion valve being the wrong type.
The thing is, we've replaced two other outside units, one a bit before and another at the same time as the one in question, all of the outside units are the same, and all of the air handlers are the same and we haven't had this problem with the other two units.
Thoughts or suggestions? We're looking to have someone else come in and look at it, but just wanted to see what the interwebs had to say about it.