The pedal having a lot of travel and a mushy feel is usually because there is air in the system, maybe you have a small leak somewhere that allowed air to get in(?) Try bleeding the brakes and see what happens... As for that repair shop... did they acutally change your master cyl? If I understand the system correctly a plugged vaccum line to the booster would make the brakes very "heavy" feeling since there is no power to help you stop, your foot has to provide all the power.... Good luck.