BRAKES losing presure! any ideas

[sbsisco]

Has anyone had this problem and/or have any suggestions for a fix. My 06 H3 (25k miles) looses break pedal under light breaking but not under normal to strong breaking. It normally has excellent pedal but every now and than under light presure it will quickly drop half way to the floor..? I''ve just recently discovered that at highway speed I can get it to do this by lightly holding the pedal with my left foot and with in a small amount of time it''ll drop, if i ease up just a little and reapply it''ll drop a little further and if I stay at it I can work the pedal all the way to the floor. No warning messages, no leaks, plenty of fluid and excellent pads...Of corse it won't do it when its returned to the dealer so I've got to trouble shoot this one on my own due to the lack of quality shops/mechanics in my area to trust working on my truck. I'm plannig a trip to sw colorado next month and bad breaks on black bear pass are not a good combo...any ideas, my next move is to bleed the system in the morning just to see if it could be something that simple...but i have a hard time believing this will help but i guess it could have left the factory with air in the lines???