Brake Pedal Goes to Floor When Starting Up

On very cold mornings, teens and twenties, my brake pedal goes to the floor with little if any resistance when I first start the truck up.
2003 Tacoma 4X4 Stock everything, daily driver
It is not every time, I can go 2 to 5 days without it happening
It only seems to occur when it is bitter cold out.
Engine RPM is well above 1K
After the first plunge to the floor, it will take 1 or perhaps 2 pumps to have pressure and the pedal has normal resistance
The master cylinder was replaced by my mechanic. The problem never happened with my mechanic who thinks I am halucinating
It has happened once while driving, very scary to have nothing on the pedal when coming off of the Mass Pike at speed. Yikes!
The brakes feel normal (for a Tacoma) after that first drop to the floor.
It's almost as if there is no power boost or vacuum assist but the RPMs are up well over 1K
What can I look at?
What can be replaced?
What can I go back to my over paid mechanic with to have him fix?
Has anyone else seen this problem?
Should I go to the dealer? Which I hate doing!

 

1st welcome to T/W
now are you looseing brake fluid
when you pump up the brakes does it go to the floor slowly
did anyone look at the wheel cyls

 

There is no loss of brake fluid. 1st thing I checked.
When I pump up the brake 1 or 2 pumps, the pedal does not go to the floor. I can then drive around Boston for hours and not have an issue. The brakes feel good and there is nothing wrong with the pedal.
I am assuming that my mechanic took a look at the cylinders.

 

it could that there is air in the brakes from when the master was replaced

 

The pedal going to the floor is why I had the master cylinder replaced. We thought there may be leakage between the primary and secondary piston. Replacing the master cylinder did nothing.

 

what are the odds you got another bad m/c?i no the odds are low BUT it does happen.just a thought.