While sitting at stop lights my brake pedal slowly moves to the floor. I have now noticed the pedal wants to go towards the floor while stopping.

There is less brake fluid evident in the master cylinder reservoir and I see no leaks.

Is the master cylinder the problem?

 

Could be. When you pump the pedal does it get firmer? If so this is indicative of air (or moisture) in the lines. After pumping up the pedal, hold pressure on it. Does it continue to sink? This is indicative of a bad master cylinder.

On these newer trucks with ABS, we gotta be a lot more careful about not getting air in the lines. You can get air trapped in the ABS deal and it's almost impossible to bleed it out. I got air in the ABS thing on my '88 once and had a mushy brake pedal for the next 3 years until I traded it in.

 

ABS doesn't make it harder to bleed the air out, you just have to sometimes follow a different procedure than normal when bleeding it. As for the brake fluid loss, if there are no external leaks (lines, wheel cylinders, etc) it is possible that the master cylinder primary piston secondary seal is allowing brake fluid to leak into the vacuum booster. The fluid is then sucked into the engine and burned. Check for wetness or bubbled up paint under where the booster and master bolt together, also pull the vacuum hose off the booster and look for signs of wetness in the hose. If there is any you need to replace the master and it would be a good idea to replace the booster as well.

-Jon