Brakes slowly, but completely fade away while driving longer distances

Hello, I'm really hoping you guys can help me as I am pretty stumped on this one. I have a 2002 Mazda Tribute that the brakes completely fail on as the car drives on longer distances.
The problem started last Sunday when out for a drive that was approximately 35 miles, the brake pedal became very spongy and eventually went all the way to the floor with no stopping ability at all. I managed to limp it home and continues on with what I had to do that day. Three hours later when I went to move the car, I had complete brakes again, as if nothing happened.
While I had no brakes at all, I checked the brake fluid level and it hadn't lost a drop. I also looked underneath for dripping brake lines and wet spots, and there was nothing.
Thinking that I may have a bad master cylinder, I replaced it with a brand new one. I bled the entire brake system, but the pedal was still spongy but it had been a little spongy for a while so I really paid it no mind.
Tonight, 4 days after changing the master cylinder, while on an approximate 40 mile ride, upon arriving at my destination, the brake pedal became really spongy again and eventually I completely lost brakes again. Again as before I have lost no brake fluid.
One thing I have noticed is that when I go to pull out of a parking space right before this happens, the car does not want to roll freely. My transmission is pretty strong and I have no trouble ordinarily just putting it in drive and coasting for a bit to back out of a parking spot but I have noticed on both occasions that the car doesn't want to move on its own. I have to hit the gas pedal to get it moving.
I have a few more things that I want to check out, but I could really use an expert's opinion on this one. What advice can you give me, besides trading it in because that has crossed my mind lol

Thanks,
Marc