loss of air brakes
 

I recently had my brake pedal sensor changed and immediately i noticed my brakes werent working right. Its very hard to brake and sometimes i have to use both my feet. I dont know if i have to bleed my brakes cause there might be air in the lines but i hope i dont need a new brake booter. They arent expensive but the labor definitely is. I have a 99 s80 2.8

not sure how a brake pedal sensor replacement would cause that to happen unless there's an adjustment for the pedal height. Did you look to see if anything is binding the pedal arm's action? did you go back to the shop who did the work to see if they can inspect what they did?

As you your other comments, sometimes when the booster goes you can hear hissing. If you have air in the brake lines (changing the sensor wouldn't open up the system so its not related) usually you can pump up the pressure with a few quick pumps to change the feel. Other causes for a hard brake pedal could be a vacuum pump issue or the vaccuum lines feeding the booster, or possibly a problem with the master - but again nothing changing a pedal sensor would cause.

To listen for the hiss does the car need to be running or not?

hiss is when you press the brake pedal and their is some vacuum so try with engine running. I still doubt its related to the booster - I'd be looking for something binding the pedal...