Air Brake Disc Rotor problems of the 2000 GMC Jimmy

1st 3 time now my check engine light went on for the samr thing secondary air injector system malfunction. It was just a bad sensor its still doing it. They want 140. Just for parts to fix it. 2nd I spent about 1000. 00 to fix frount brakes rotors pads andwheel berrings. The owner of the garage said that he fixed about 50 of these a year my SUV was only 2 at the time 3rdi also spent almost 3000. 00 on a tranny last year because I lost all gears I only had drive. I had recomened the GMC Jimmy to at least 8 to 10 friends 7 bought them 5 have had the same problems as I have with tranny and brakes now my brakes are squeeking frount and back not grinding nothing wrong with back brakes ever had them checked they said they are fine. But very squeeky that is very annoying. When we lost reverce we was backing up and if the car behind us didnt back up we would have hit him. I thought I woul let you know some of the things that have been happing to myself and friends the last thing we fixed was the tranny.