This hapended to me. The Banjo bolt has 2 copper crush washers either of the Banjo fitting. You should use new washers if replacing and only cost pennies to buy. - Make sure you get the right size.
Do not over tighted when fitting the bolt back since they can shear real easy since they are hollow at the thread end.
They do vibrate loose if they were fitted without the correct torque or if old washers were used at the time.
Nik