2005 Toyota Sienna Rear Evaporator Leak

Is the A/C system fully charged with the proper refrigerent?

yes it does have proper refrigerant. Everything about it works great, but it leaks water out the back evaporator. The toyota mechanics have no clue, but they think it is the expansion valve.

First, you must understand this, your evaporator is not leaking water. That is impossible! (it may leak refrigerant) but not water......
Evaorators, in an automotive A/C system, "evaporate heat" from inside the cabin, to the outside atmosphere. They do this by circulating refrigerant through the A/C system (which includes the evaporator). This refrigerant absorbes "heat" and releases it to the outside atmosphere....
Your issue, is the condensation that is a result of airconditioning.
There are many factors that can cause excessive condensation to build up on the evaporater. The most common is the level of the refrigerant.

How much refrigerant is in thre system?

EVERY A/C system has an evaporator- AND IT CONDENSATES which results in water dripping- THERE must be a water route for the evaporator- either a little hose, or a runway, something. Every car usually has a hose to let the water drip down to the ground. try looking for a runway or a hose-
every car that has a/c drips water
- try making a shield or something to direct water away.

Sounds like a great idea. I actually came up with it myself yesterday. Thanks for the suggestion.

Every A/C system indeed has an evaporator, and the condensation must be drained out somehow; but A/C systems in cars are not new, and I am sure that the Toyota mechanics that you mentioned, checked out the drain system. If they didn't then they are not Toyota mechanics. You also mentioned that the mechanics think that it is the expansion valve. The expansion valve meters the amount of refrigerant that is allowed into the evaporator. If too much is allowed to go in, the evaporator will create too much condensation, which will - in turn - cause the drain to be overwhelmed with water...Re-engineering a new drain will not fix the problem.

Okay, so I have the same situation as the gentleman who posted this original post, with a small twist - it does leak water(condensate) from the same point on the evaporator that he showed in his photos, but it also drains from the drain hose that goes through the body of the car to the outside. So, it appears to be draining both properly and improperly at the same time.

Thoughts? I noticed you asked twice about the level of refrigerant, but I got the impression that only if the amount of refrigerant was high would you end up with excess condensate. My van has 140k miles on it, and while the rear AC is cooling fine I wouldn't guess it somehow got an excess of refrigerant, unless it's connected to the refrigerant up front. The front AC was just worked on, and had to be recharged.