Air Conditioning - How to find a Central A/C freon leak

Brand: 5-ton Goodman A/C (10 years old)
About a month ago, my A/C stopped blowing cold air. I called an A/C tech. The tech determined that the freon was low by 4 lbs. He did not find leak in the outside unit. He suspected that the leak might be in the A-coil. After spending $250 for 4lbs of freon, the system worked well. However, two weeks later, the A/C stopped blowing cold air again. Called another A/C man and verified that the leak was coming from the A-coil. Replaced the A-coil and the system was recharged. The system worked great for another 2 weeks and stopped blowing cold air. I called the same A/C man to come out to look for the leak. Using the electronic leak detector, the tech spent more than an hour inside (A-coil) and outside(condenser & compressor). The lines were also checked. He could not find the leak anywhere. If I am losing 4 lbs of freon in two week, shouldn't the leak be easy to detect? Any advise would be appreciated!

I would try a new technician. I would also agree a leak that big should be able to be found quite quickly.

Besides the electronic detector sometimes die is also added (by a technician, refrigeration work can't be done by homeowners) so that a leak can be spotted.

From other stories on here, I've also read of the Schrader valve leaking, and techs not being able to find the leak because their guages sealed it.

Good luck.

I also agree. A leak that big should be easy to spot. Also it would be likely that there would be oil at or around a leak that size too. Did the guy who replaced the coil show you where the leak was because it seems apparent the coil was not the problem?
Sometimes outdoor leaks can be tricky depending where they are and if it's windy outside can make it more difficult. But that size leak would be easy to spot with soap bubbles.
If you put a large plastic bag around the condenser (with it turned off of course) and let it sit for a day, an electronic leak detector would have no problem if your service guy stuck the probe in the enclosed bag. Maybe that would at least localize the leak for you.
I assure you, the leak can be found.

i had a leak that was a devil to find. i found it finally when i bumped the freon line. it would not leak, in a certain position.
now i beat the pipes when ck for a leak. only gonna trick me once!!!

The first tool you use is your eyeballs. As said, you look for oil, oily dust in all accessable parts of the system. Second tool is your ears, 4#'s you may here it.

Next is the electronic detector. Three hits in the same spot, break out the soap bubbles. Don't forget to check schrader valves, stinks looking for a leak that you have sealed with your gauges.

Still can't pinpoint it, inject dye and return with UV light. Dye mixes with oil and will glow neon green under UV light.

You return a day or two later and check with the light. No green sign. You start to get ticked off.

Two options. One is to recover the refrigerant and pump the whole thing up to 350# nitrogen and listen, and use electronitc again. Another is to cut the system up. Isolate and charge evap, liquid line, suction line, and outdoor unit with nitrogen. Which ever leaks is the culprit.

Lineset usually makes a straight run up the side of the house, any siding work or picture hanging in that area lately?

Yup, do everything you can as Jarredsdad mentioned that is in the realm of DIY for A/C leak searching. If that technician can't do it after all, try someone else, maybe even ask about their methods. It can be found!