07 Malibu LTZ issue with brakes
 

Ok so for a couple weeks now I've had a vibration or pulsating feeling coming from what I believe is my brakes. Rear brakes were done a month ago btw. The shaking or vibration was very simular to when I had a caliper lock up. Some days my brake pedal would feel normal and on those days the shaking was minimal. Other days the pedal was very stiff, barely had to touch it to stop. We'll today driving home I stop and feel a pop in the brake pedal which proceeds to go to the floor after the pop. Brakes are now very spongey. They work but only when pedal is to the floor. The shaking stopped as well. I have checked the lines from what I can see. Took all tires off and didn't see and fluid leaking. Actually I haven't been able to find any brake fluid leak. Could this be the master cylinder? What would that pop be from? If a brake line broke wouldn't fluid be leaking somewhere under the car? Thanks in advance