I've never experienced something like this before. So far it's happened twice from what I remember. I was driving out of town maybe 45min on a back road to the next city over after some spirited driving we arrive to the city. Next thing I know it I go to hit my brakes and the peddle is really soft and almost goes to the floor after hitting it a couple times helps make it stop a little better but it's still soft. I don't think the brakes never got hot enough to boil the brake fluid and that would show it's self during hard use of the brakes not after driving a little ways. But I will let the car sit about an hr and then start it again and the brake peddle is right back to how it used to be and works great, which makes me feel as if it's not air in the lines but there could possible be but then I feel like it would be soft all the time. Both times this happened it happened when I was arriving to where I was going lucky so nothing bad happens but it scares me so much, and I have no idea why they would come back and not continue to be soft. It feels as if someone cut a brake line almost and fluid is just being pushed out. Yes brake fluid is full

 

Not sure if anyone will run into this problem since it was super strange. But I ended up replacing the brake lines with stainless steal lines and dot 4 fluid and have't had the problem since. Maybe I ended up getting air in my lines from boiling fluid but my peddle never felt spongy but air in the lines is the only thing I believe it could be