A friend came to town with a circa 2000 Chevy Cavalier, 135k miles. Before she embarked on the long road trip that brought her here, her husband checked the fluids and put on new brakes for her. This car has disc brakes and the front and drums in the back. At first, it was fine. But after some time driving on the highway, the braking became excessively spongy, to the point where it did not feel safe. She had to push the brake into the carpet to get the car to stop.
I drove the car around for a couple miles, and could not reproduce the problem, even with heavy brake use. The breaks were good and solid all the while I drove. I could produce another problem, that has also occurred since the new brake job; the rear passenger side drum break squeaks once warmed up and braking pressure is applied. I’m doubtful that that is related to the brake sponginess, but thought I’d mention it.
I checked the brake fluid; it’s full but green, indicating that it hasn’t been changed recently. But it’s a light color green, not dingy, and there’s no off smell.
She has to leave in the morning, back out on the road, and I live in a small town with no Saturday mechanic. I suspect a failing master cylinder, but I’m not confident that’s the problem. I’m advising she should stop in the next town with a mechanic, which is going to be about a 25 minute drive.
edit: I should clarify that I’m asking: does it sound likely that this is indeed a bad master cylinder? And a 25 minute drive should be reasonably fine? (in other words, there’s nothing here to indicate imminent brake failure?)