I would have originally said it was air in the brake lines but you said that you had your mech bleed them for you. so now i'm agreeing with tegbooster about a leak, probably somewhere around your master cylender, mabye they didn't connect the new one to your cables correctly or the device itself is leaking.

not to dis your mech but mabye he didn't bleed the whole system enough or correctly and there is still air in your lines. Because he replaced the MC (which is at the very begining of your brake line) he may not of ran enough fluid through the lines. or mabye he is bleeding your brakes in the wrong order which could result in traped air in the system.

so if you can't find any leaks around the linkage of your MC, or a problem with the pedal set up, then i would recomend bleeding them yet again.