To begin with this is pertaining to a 99 SR5 V6 4wd. When it is wet out or when the brakes are cool they sometimes go to the floor and i can kind of feel a vibration like somethings slipping. They also occasionally do it when i turn into a driveway with a downward slope and apply the brakes. Checked the pads, rotors, and fluid today and all looked good. Any ideas?