Brakes

Have a two wheel drive SUV (listed above). Recently I had been driving and I know this has nothing to do with it but when it started. Driving had the air on turned it to max went to stop at a light and pedal slowly went to the floor, kicked the air down did not do it again. This week was crossing a bridge as I was coming down stepped on my brake heard a clank sound and pedal went to the floor. I made it home next day no problem. Few days passed started my car got to the end of my drive and pedal went to the floor, so I released it kept on driving it was fine. So this afternoon car sat all day after parking it in the shade went to move it pedal started slowly going to the floor again as I was backing up, so I put the emergency brake on and tried to move backwards it would not move. After unlocking it parked it and before bed figured I would try it, same thing again, but this time tried the parking brake but the car moved this time. So it seems to have brake as your driving it only seems to mess up parked at a light or stop sign. The fluid is full. I had brakes put on about five months ago front and back. There is no lights on saying ABS or anything. It has discs front and back. I did look underneath it on the drivers side back underneath on the backside what looks like a plate the caliper bolts on looks like it had been wet covered in sand but not wet to the touch, no obvious drips or leaks. I am scratching my head on this one there is also some kind of sensor on the master cylinder. It is a v8, Tritium engine and I need help guys this grandma raising a three year old grandson and I need my car. If someone can give me any advice I would sure appreciate it.