I've got a problem that seems to be getting worse. The majority of time, the brake pedal is hard like normal. But lately, the pedal has become soft at times. It is very inconsistent, but seems to happen more when the motor is cold. Whenever it is soft, I pump it once and it is back to a hard pedal. It has nothing to do with how often the brakes are used or how long its been since the last time I pushed the pedal. This morning, it was so soft the pedal did not want to return to the up position. I stuck my foot under it and pulled it back up, then push it back in and it was back to being a hard pedal again. I have found no leaks or anything obvious. What can be giving such an inconsistent problem?