Hey guys, my wife has a 12 Camry SE, 154,000 miles, a couple of days ago she was coming up to a stop sign, and the brake pedal sunk to the floor. She pumped it a couple of times and it went back to normal, on the way home she said it happened 1 or 2 more times. I checked for leaks on all 4 wheels, nothing, same for the MC no visible leaks. Fluid still looks good, I'm thinking the Master Cylinder, if someone else has some insight as to what might be going on here, I'm all ears, Thanks

 

This could be a couple of things but I think you are on the right track.
Was/is the pedal spongy feeling?
Pedal is hard?
I'd try bleeding the brakes just to see if there is air in the lines and rule it out. Sometimes air can get in and not show signs of leaks.
I guess start with something relatively cheap to check.