I had this exact problem 2 years ago. I had a mechanic friend of mine tell me that air must have leaked in while sitting and I should bleed each brake a little to find the culprit.... so I did. It sort of "fixed it ....NOT". I decided to empty the master cylinder bowls and add new fluid (Pentosin super DOT4). I bled the entire system starting with the rear farthest from the master cylinder. I thought about buying self bleeders but with the help of my fiance all went well. All new fluid and problem solved.