It's an R-134a system (according to the internet, R-134a became the dominant refrigerant in 1994.) I refuse to put stop-leak into it. That stuff is notorious for crippling systems... a $15 bandaid could change a $300 repair into a $800 repair.
Dan is coming over Wednesday. His Yelp reviews are all extremely positive, and I do love not having to leave the house!
You're right about Leak Stop, depending on the exact cause and location of the leak. The negatives outweigh the positives if you intend to have the AC serviced in the future. If, however, you are just trying to help the current AC blow cold air for as long as possible so you can delay the inevitable full system replacement, then the product does have some merit.
If the truck is from 1999, making the AC 10+ years old, you may be in the ballpark range of the AC's normal lifespan. I've had a 1989 Plymouth Voyager minivan need the AC replaced with 8 years of use. I might have gotten by with a condenser replacement, but since the newer standard was mandated, I went ahead and replaced the whole system with R-134a. It was explained that the old refrigerant was almost cost-prohibitive, as you needed special certifications and licenses to even order it, and there were special requirements for storage and training employees to use it. It was becoming almost impossible to find on Oahu, so I opted for a retrofit.
I've had 2 ACs fail within 10 years of purchase. All my other vehicles have gone 12-14 years with no AC problems, just routine maintenance and service when they stop making cold air.
Then again, we are talking about a Ford here, aren't we?