Be careful!
Brake fluid doesn't just evaporate. If you refilled and bled the system and did not find a leak or failed component, you have not identified the problem. I had the same issue with my wrecker. I filled the fluid, bled everything and had no issues and no leaks. Then suddenly, no fluid and NO BRAKES. I ended up replacing all wheel cylinders, M.C. and airpack. I found mulitple wheel cylinders with small leaks that did not produce any external signs of leaking
You need to be sure your truck will stop. For your safety and the safety of those around you.