MRSUIT

I have a problem that seems to be getting worse. When at low speed in reverse the brake pedal will vibrate and sink to the floor. I did have the booster recall done which was just the reflash. Say your parking and you back up slowly and then when you go to stop so you can put it into park or drive when I go to stop it ( almost always now ) the pedal goes loose and as it’s sinking to the floor it vibrates. This has happened 1 time in forward which was also when parking. Other than that it is in reverse only. Does this sound like a vacuum booster issue or any ideas ? Any help is greatly appreciated