There are two power brake versions available. One has the common and reliable vacuum booster. There is no way to monitor its performance with a warning light. The other system is called "hydro-boost" and runs off pressurized power steering fluid. That system was developed for use with diesel engines because they often don't have enough vacuum to run the regular system. They might have used it on your truck for the same reason.
Also check the brake fluid level in the reservoir. If it's just half full, you can try adding a little from a clean, sealed container of new fluid, but don't fill it to the top. There needs to be room in the reservoir to accept the fluid that will return there whenever new front brake pads are installed in the future. If the level is REAL low, suspect a fluid leak. Be absolutely CERTAIN you do not get the slightest hint of power steering fluid, or any other petroleum-based product, like engine oil, mixed in with brake fluid. Don't leave any container of brake fluid uncapped for very long as it will such moisture out of the air. If all of this seems okay the booster needs to be replaced here are instructions below. Check out the diagrams (Below). Please let us know what you find. We are interested to see what it is.
Images (Click to enlarge)
Friday, July 15th, 2011 AT 7:18 PM