soft brake pedal on a 2008 Toyota Tundra 5.7l engine

On my 2008 Toyota tundra, when you first start the truck and push on brake pedal it goes almost down to the floor. You have to pump the pedal 3 or 4 times to get brakes. What might be the problem?

 

You may have air in your brakes, or there may be some water in there due to heating and cooling cycles in the braking system. It's what I've heard, never seen it personally, but I always crack the bleeders to reset brake pistons.
Maybe start bleeding your brakes, check for leaks within the piston of your calipers, check your lines to the calipers, check for leaks and check your reservoir.