brakes hissing?

my 99.5 jettas abs light has been on for over 2 years no big deal. I can live without abs. Recently the red brake light started flashing so I took to the volkswagen garage they scanned and said my two rear abs sensors were bad. However I recently noticed there is a hissing noise that sounds like its coming from my brake pedal when I push on it. I just changed my rear brakes pads and rotors myself I am concerned that I did something wrong. Could this noise have to do with the abs failing I dont see how putting in new sensors will stop a hissing noise.. Also I am worried I got air in the lines when I pushed the rear pistons back in or something.. :banghead: please help :confused: