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?
I doubt the rear wheel cylinders will go bad. As far as the MC, it sounds bad. If you forced the caliper pistons in pushing the brake fluid into the MC, the seal on the plunger may have been damaged. Thus, the MC is bad.