soft brake pedal 2005 monte

after car sits over night. the first time you apply brake pedal it is real soft and brakes do not seem to grab good ,next time you apply brakes they seem normal and work good for rest of the day.even after the car sits during work approx 8 hours.????????????? iam i losing vac to something over longer period of time .

ps does anyone read these post and ever reply I need help asap

thanks all