GM-GMC: 2000 GMC Serria Truck 4wdr Small V8, gmc serria, fluid leaks

Question
My brake pedel goes all the way to the floor. Truck stops, but only when peddle is at the floor. Brake fluid in master cylinder is ok. No fluid leaks that I can find.  No brake warning lite. No abs lite. Saturday brakes were ok, Sunday not ok.  No indication of brake problems on Saturday.

Answer
If you are certain that there are absolutely no fluid leaks, I suspect your master cylinder has failed.  Before you condemn your master cylinder, you should double check your braking material first.  A seperated brake pad or something similar can also cause the concerns you have.  Pull your tires off, pull your calipers off and have a good look at the pads etc.  If everything checks out, and again you are sure you don't have any fluid leaks, replace your master cylinder.  One thing to note however, after replacing it and bleeding your brakes, if the pedal isn't feeling right, you may need to take it to a dealer and have an automated bleed procedure done on the abs system.
Hope this helps