Brake Master Cylinder problems of the 2010 Toyota Tundra

2010 Toyota Tundra crewmax, current mileage 16,752. For the last month or so while the vehicle is stopped, waiting for a light etc. With my foot on the brake pedal, the pedal will slowly go all the way down to the mat. The vehicle still remains braked, even when the pedal is all the way down. Release of the pedal and immediate reapplication of the brakes results in an initial normal pedal, followed by the pedal slowly going to the floor. When the engine is off and the brake pedal depressed, no matter how much force is applied, the brake pedal does not sink to the floor. So far this does not appear to have had any adverse effect on the normal braking. Took the truck in to the dealer, they test drove it, said they had it happen, but that Toyota brakes were different and that they had another Tundra in the shop which did the same thing. They also said that another Tundra owner had complained of the same issue and the Toyota rep had checked it out and declared it normal. They then told me there was a 2nd recall on the accelerator pedal and requested that they perform it. I agreed. Left the dealership with the paperwork stating "no problem found". I would assume it would be normal if the brake pedal went to the floor from day one, it didn't. I would presume the brake master cylinder is fine as the pedal is solid with the engine off. Maybe a seal leak within the abs block? no apparent brake fluid loss, the reservoir has not dropped.