GM-GMC: 1999 CHEV 3500HD BRAKE PEDAL..., competent mechanic, brake fluid

Question
1999 CHEV 3500HD BRAKE PEDAL GOES ALMOST TO FLOOR TO STOP TRUCK   HAVE BLED BRAKES SEVERAL TIMES WITH NO DECREASE IN PEDAL TRAVEL  THE TRUCK WILL STOP BUT IS VERY SCAREY WHEN YOU HAVE TO PUSH DOWN SO FAR

Answer
Some part of the system is leaking or the master cylinder is bypassing and cannot compress the brake fluid to build enough pressure to make the brakes work properly.

This is a very dangerous situation and may cause an accident if it is not corrected.  If you cannot find the problem, take the vehicle to a competent mechanic before you lose the brakes completely.