Complex sentence: His re-design of reed blocks and chambers eliminated the historic issue of "dead air" gaps and poor reed response and his collaborations with the Canadian builder Leo Niemi developed tunings that held for years, not months.
Split into simpler sentences: His re-design of reed blocks and chambers eliminated the historic issue of "dead air" gaps and poor reed response. His collaborations with the Canadian builder Niemi developed tuning innovations that allowed the accordion to hold its tuning for years, not months.