This might be a bit of a red herring but - the servo for the brakes are air operated. If you loose brake oil then the pedal goes squashy i.e. it's compressing air in the oil lines not oil. While the pedal is hard to press that suggests that there is still oil in the system, just no servo assistance. Now, the servo takes it's air pressure from somewhere and I wonder whether it's linked to the crank breather. If that is the case then by overfilling with engine oil you may have blown the crank breather hose off the block which will then allow the excess oil to escape. On carbuerreted engines the breather would go to the carbueretter as the waste gases in the engine block would help with combustion. I don't know enough about Fuel injection engines.
Anyway, have a look and let us know.