I decided to get a better look at things and take action!!! The thermal wrap is easily removable with scissors or rubber-gloved hands right along its seam and so I began removing it. Be careful of the rubber hose that is part of the a/c line above the muffler behind a crimped fitting. What a UV dye mess in there! The pin hole leak on the low side suction tube is shown in the pic and was immediately apparent after attempting a fill as residual r-134 and oil/dye were still escaping at the time.
Indeed, as others note in this thread, Ford's design of a loose fitting slip-over thermal wrap allows for gravel/sand/dust to build up between the wrap and aluminum tubing causing wear and exacerbating rock strikes. Mine had quite a lot in there. A properly designed heat shield could have offered the same radiant heat barrier from the exhaust as well as flying rock strike protection without the sand/gravel build-up flaw, but I digress.
Given the small hole size I decided on an epoxy patch. Soldering, brazing seemed like too much heat close to the rubber o-ring fitting 6 inches away, and the flux would go in the hole, etc. There are a lot of off-the-shelf epoxy products with magical marketing claims out there. Avoid them in my opinion. ALUM BOND is the product I decided on that seemed most appropriate with specific claims about its ability to bond aluminum tube in A/C systems. You can't buy it off the shelf in my area, it has to be ordered and shipped (took 1 day). Its made by a manufacturer of HVAC equipment, is designed for aluminum exterior A/C applications, and seems to be praised by HVAC repair personnel in anecdotal comments for this type of pinhole fix.
I removed all the crummy thermal wrap and started cleanup with denatured alcohol. I covered the pinhole with a small circle of tape while doing this and sanding around the tube. 150 grit SC paper or cloth seems to work well. A quick shot of refrigerant allowed for gas to be squirting out while I sanded around the hole, hopefully preventing system contamination. Some more quick wipes with denat alcohol to clean things up and a quick tap on the schrader valve to release any residual pressure, ready to go. Here's what it looked like right before I applied the epoxy compound:
Mixed up the 2 part epoxy well, calls for 2 minutes of mixing. I did 4 stripes in the bowl A/B/A/B and mixed the crap out of them until uniform and then did a quick thin coat application everywhere I wanted it. Then after some more mixing and minutes of time it had thickened and I applied enough to wrap the tube thoroughly with some thickness to it. It can be tooled with a wet glove and ultimately I was using a wet paper towel to tool it around a bit where it sagged/dripped during set time. The hole is on the rear-facing side so I wanted good thick coverage there. I had to come back to it several times over the course of 90 min - 2 hrs and make sure it was how I wanted it to be. Here's what the fix looks like the next morning:
I'm pretty happy with the way it turned out and am confident it will hold. I'm waiting on a pump and thermal wrap delivery to finish the job. You'll HAVE TO REPLACE the thermal wrap especially on the rubber hose above the muffler!!! It rained like cats and dogs here over the last few days so I'll have to pump down for 24 hrs to be sure of water evacuation. I'll follow up and let you know how things go. I hope this helps somebody in a similar spot with a similarly small hole to fix on a real long run of a/c tubing.
To be clear, if this doesn't work, I plan to use the aftermarket autocoolingsolutions flexible hose replacement kit shown
at this link. Replacing the hard lines looks to be a giant pain ITA as you follow them through from the muffler to up behind the engine near the firewall. With the flexible ones, you'd just thread them through easy peezy, but we'll see how this vacuum holds and go from there.
***If this hasn't happened to you yet, please consider REMOVING AND REPLACING the sand & gravel-collecting thermal wrap above your muffler***