Slip light and brake failure

Hey guys, I recently purchased a used 2008 Nissan Titan SE and have been having some unusual brake issues. I was driving one day and pulled up to a stop sign and noticed the brake pedal sank to the floor and I wasn't sure I was going to stop in time. I noticed the 'SLIP' light come on, and it stayed on until I turned off/on the truck. I notified the dealership and they wanted to look at the truck right away. I brought the truck in and they couldn't initially figure it out so they brought it to the Nissan dealership. The Nissan dealership said the proportioning value needed to be replaced. My car dealer replaced the proportioning valve and master cylinder (as a precaution). All covered under warranty, I was a happy guy. Then I was driving through the Green Mountains and a guy in front of me decided to make a left turn without a turn signal and I had to put some real pressure on that pedal. I was probably going 50MPH and the pedal sunk to the floor. I was able to avoid a accident, but it was unnerving. If I'm going 45 or less, the brakes 'catch' and stop me as I would expect, if I'm going any faster, the brakes don't catch and the pedal sinks to the floor and I start praying to god that I don't kill anyone or myself.

So far, the master cylinder, and proportioning valve have been replaced. the brakes have been bled, the brake pads and rotors are good, whats wrong with these brakes, I'm nervous to drive it.