Complex sentence: Barney thinks he can stay in the past as long as necessary, but first Ruf backs out and has to be returned, and then the time machine itself breaks down in the present day, taking hours to repair.
Split into simpler sentences: Barney thinks he can stay in the past as long as necessary, but first Ruf backs out and has to be returned. Then the time machine itself breaks down in the present day, taking hours to repair.