Sinking brake pedal!!!

My 2015 Tundra is a beautiful speciman. Unfortunately, I'm a second owner and had to do some work. New front caliper, new rear disk brakes, and a new master cylinder. I have a nice comfy pedal until I start the truck. Then it goes away. I've bled the hell out of it with over a gallon of brake fluid. I have good vacuum and no leaks in the booster. Ive done the abs bleed. i dont have any lights, codes or errors. Ive slid in dirt. I've slid on roads. The truck stops. I dont have to pump it. It just acts fine but I have the ability to crush it against the firewall. What give folks? I have been wrenching proffessionally for a long time and haven't run into this before. I'm certainly not looking to be a parts changer on this.

 

Any time this has happened to me I had to slam on the brakes a few times and then bleed them. (I mean drive it and slam on them like a kids ball rolled in front of you)

I am sure it was ABS but dont know.
It seemed to fix the problem though.

You are bleeding all four, right?

 

Yup. A good, solid bleed going from far to near. I'm gonna hook up ye old snap-on to it this weekend. Im not expecting much as I have already done this prior to the mc. I assumed it was the mc after the unsuccessful bleed.