Interesting dream theme; interesting that it has been so extensively recurring.

I'm no expert, but I do dream a lot and have been keeping a dream journal for over 30 years... started as a homework exercise when I was in counseling and I never stopped.

What I would be looking at is that the type of car is uncharacteristic for you, and the brakes are not working. Can you find any parallel between something "uncharacteristic" going on in your life... and occurrences of this dream? Seems like a subconscious caution if you're agreeing to something outside your normal realm.

And yes, I've had car dreams with failing brakes, although mostly about climbing a hill, and the engine cuts out and the brakes fail and the car starts rolling backwards.