Occasionally when I push on the brake pedal it starts to go down then suddenly feels like a grind and lifts back up as if brakes aren’t engaging. Then I put more pressure and it goes down again. The first time it happened it scared the dickens out if me that the brakes had failed and I stomped on the pedal and of course jerked to a very fast stop! This is a car I bought just as a temp car and obviously has lots of issues and I don’t want to put a lot of money in but I also don’t want the brakes to fail on me. Any clues hat it might be? It’s a 2006 Buick Rendezvous.