What would cause the brakes to feel really soft and almost go to the floor? on 2008 Buick Enclave

On occasion, my brakes will get really soft and almost go to the floor when pushed. It will scare you to death. Then, all of the sudden, they are fine. Just every few weeks it will do this. I thought I was imagining this at first, but now my husband drove it and noticed it, as well.

Asked by for the 2008 Buick Enclave
Do yo know a good mechanic in your zip code! If so, test drive is what it needs to determine the problem! Mechanic will know how to check for a master cylinder bypass problem by lightly and constantly applying brakes. Any warning lights on in dash?
OK! That's all i need to know!!!
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1 more answer , 3 more comments
You might have the ABS system function, auto bleed, to make sure there is no air trapped in ABS controller. The TECH2 scan tool has this function.
I apologize, but what is the tech2 scan tool?
Many shops have them, I have one. It is the scan tools that the manufactuer uses for the GM line of vehicles.
Oh, okay. Thanks!