I'm having the exact same problem, and I've replaced the brake booster and the master cylinder w/o luck. Also, the pedal assembly is not cracked/torn. A mechanic friend of mine mentioned it likely is the front caliper pistons binding in my case. The pistons are likely worn and rocking back and forth in the cylinders a bit and binding. Not sure how if it will apply to you, but it's another suggestion. When my brakes bind, the pedal kinda "pops" suddenly once I've applied enough pressure to get passed the bind, and I think there's a noise coming from the front axle.