My initial impression is head trouble (especially with the stop leak)
Are the people working on it knowledgeable about late model vehicles, or just good with their hands?
If you fill the system up, you must then "burp" it. (when it gets to temp, the thermostat will open, and an air pocket will be released)
Once you have the system Totally full, then put a piece of cardboard under it, and watch for leaks.
If you have no external leaks, and you still lose fluid, you have bigger troubles...
(did you buy it used, and how many miles, then and now?)