Trust me, trust me, the problem with your brakes is in the mastercylinder. There may be nothing wrong with the master cylinder itself, just there's at least one air bubble trapped in there that causes the brakes to have lots of pedal but very little braking . I just solved this probelm myself, thanks to my son who suggested cracking the individual brake lines at the master cylinder and bleeding each one in turn right at the cylinder to get the air out. If your mechanics have a pressure bleeding device they might acheive the same thing.
Bleed, bleed, the master cylinder. Don't buy anything new until you're certain that they did that. If necessary stand right there and make them do it while you watch, they're going to be very embarassed when a female sovles their problem.
Put your foot on the pedal, press hard and long, if the pedal eventually goes all the way to the floor, it's air in the master cylinder.
When they eventually do it your way and bleed the master cylinder make this test again, if you now have brakes but the pedal eventually goes to the floor, bleed it again.