Check to make sure the brake pedal is returning all the way back as it can slightly be applied and the slight drag of the brakes will heat the fluid and it is not able to return to the master cyl since the brakes are slightly applied and the extra heat causes the fluid to expand and make the calipers apply the pads. After a while of sitting the fluid cools off and returns to normal volume and the brakes release themselves. If the pedal is returning fully you need to get the system looked at by a professional since braking system failure can kill you. Plus, that drag is killing your gas mileage and uses valuable horsepower. Good luck with it.