How rusty can functional brake rotors get?

I brought my subaru in for a brake line leak about three months ago. The leak was fixed, but the brakes were still a little spongy and needed a pump or two to achieve full pressure. Now, about two weeks ago the rear brakes started grinding. I assumed it was the pads getting worn down. I looked at the rear brakes and the passenger side rotor was completely rusted over. No clean metal at all & it was thick, heavy rust. The driver side rotor was also very rusty, but there was at least a little bit of clean metal. Two days ago, the passenger side inside pad fell out and it blew the caliper piston.

Now, it seems to me that the most likely scenario is that when the mechanic fixed the original leak a few months ago, they did something to the lines so that the rear brakes did not function properly, resulting in the heavy rust on the rotors. They tried telling me that rotors just get rusty like that sometimes.

Does anyone know what’s going on here?

I looked at the rear brakes and the passenger side rotor was completely rusted over.

Within SECONDS of applying the brakes while driving the rust should be completely wiped off the rotor. If you’re seeing rust on the rotor after you were driving then I suspect the caliper isn’t working properly.

It sounds like the original shop did a poor job, leaving you with air in the system and possibly other issues. I’d bring the car to a better shop and replace whatever needs to be replaced to give you a safe system.

Your rear brakes aren’t working at all. And I am not impressed with the shop you are using. Both rear calipers need to be rebuilt or replaced. Flush out all the old brake fluid too. Then test the operation of the rear brakes. If they aren’t working then the proportioning value is suspect.

If the back brakes are in that bad of shape, make sure your new shop gives the whole system a complete inspection. Having to replace brake line from rust is a sure sign there may be rust elsewhere, so have them inspect for that, too.

While the other shop may have botched the job, it’ll be hard to prove, and you sure don’t want them touching your brakes (or car) again.

Regarding the rust, are you referring to the center hub part and the outer circumference or are you referring to the face of the rotors; the part that is wiped by the brake pads.

Brand new rotors can rust overnight just from being left on the porch. I’m not too quick to jump on the shop here. How many miles on the car, have the brakes ever been serviced (other than the leak), and do you live in a northern Rust Belt area?

The rust you see on the rotor is on the outside of the rotor ( you didn’t get under the car and look at the inside did you). The brake piston in on the inside of the rotor in the caliper. Your caliper was frozen with rust and not sliding. The sliding of the caliper is the only thing that presses the outer pad against the rotor.
Your inside pad fell out because it was completely worn out from doing all the braking on that wheel.
You need a good brake job and if you brakes are sliding caliper the slides need resurfacing and lubricating and the caliper should be inspected for sticking.
If you have floating calipers, the calipers should be replaced.
Your shop didn’t do anything to cause this problem, but they didn’t do you any favor by letting you go with the brakes in this shape.
Are you a person that gets nad at the mechanic for pointing out additional problems? I wouldn’t want to be the one to tell some people their car needed work beside what they brought it in for.

This is why a good shop will take a holistic approach and not just replace what has actually reached failure.
This works for toilets too.
I bet the brake fluid looked really nasty.
How old is this car?
I would replace all rotors, calipers, pads, flex hoses and the master cylinder.
If the leak was a brake line I’d replace any other hard lines that look iffy.