Just replaced rear break pads on 2006 Buick lacrosse this is not anything new

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Just replaced rear break pads on 2006 Buick lacrosse this is not anything new to me pedal goes straight to the floor , bled , no twisted lines, everything is tight, there is one leak can't tell if it on going but it between master cylinder and brake booster , no leak under dash where break peddle is, hissing noise when press down on break when car is on , hard to stop., Did some research online and points to master cylinder or break booster but I'm also looking for why and how from a simple break pad replacement. Any help appreciated