yz front bake problem

alright i have a 07 yz250 and the front break worked goood all day i was riden at a track worked good and then i got home jus took it in our field and jus like that the break lever would jus go all the way in and no brake atll then i stopped and looked at it and if u pull the lever in really fast the break catches but let off jus a little the pressure goes away and jus sinks all the way in. so basicly if u slowly pull in the break theres nothin at all just goes all the way in and if u pull it really fast and quick it grabs and works good but then u let off a little bit jus goes all the way to the grip. i tried bleedin them the old fashin way then tried the seringe but prolly put more air doin that and now i got a power bleeder to see if that helps at all........ any one no what happend or whats wrong with it?????? o ya and in the resivour the brake fluid in when u pull it in slowly it bubbles so ya im confused