Brakes

After installing new brake pads, brake are low to floor with air whoosh sounds. Mechanic said I now need to change master cylinder, because with new pads the MC cups are stopping at a higher place, causing fluid leak. They wear high before the change with no air sounds. Sounds fishy? I asked if the need 2 be bled, "No" but he advises me, when he changes the MC the back brakes will probably blow out! Any advice?