2012 versa with approx 55000 miles. After sitting at idea or with engine off the brakes make a squishy air sound as I pump them. After I drive for a for 50 feet or so normal brake brake pedal activity starts again. This just started this week. Brake fluid is in normal range... any thoughts would be appreciated. .... I'm thinking it could be a faulty master cylinder but when I hold the brake pedal down after driving it keeps pressure and maintains its position.