I'm having two different problems at the same time. 1st, I noticed the front wheeels were dirtier than normal (Brake dust). When I went to rotate the tires, I noticed that the rear rotors are rusty, as if the rear brakes are hardly touching, and the rear pads look as though they are new. BOTH rear brakes. I thought maybe the master cylinder, but whenever that has failed, on other cars of mine, all brakes failed. I'm seriously considering replacing both calipers, but I don't want throw away money I don't have for no reason.(After a year of being laid off, I'm pretty stingy with what little money I have left

)
2nd, I've noticed that Libby seems to be 'stumbling', especially when it is hot outside. A couple of weeks ago, when it was in the 60's and 70's, i didn't notice it, and thought it had been my imagination. Now that it's in the 80's, again, the mystery 'stumble' returned. I don't notice anything on the highway, just any time I'm at idle speed, like pulling out of a drive, or sitting at a light.
The 2 issues are driving me nuts.

If anyone has any ideas, I'd welcome a theory.
Oh, and the computer isn't showing any codes.