Well I hope you don't end up with what I had, same thing happened, and we tried so many things after that and nothing seemed to work untill we changed the lines and the master cylinder.
The problem was moisture in the cylinder and some gunk from moisture in the lines. We could have just blown them clean but we prefered going with new.
I would suggest purging the whole system and the best way is to unhook all the ends and blow the lines clean and then putting new brake fluid and bleeding all the connection from back to front, and remember it's always best to change all the fluid every 2 years, because brake fluid has the tendency to absorb moisture and that ruins seals and makes spungy pedal feel.
IceMan