Complex sentence: Each piece of glass installed in the nose had to be pulled out and replaced with new distortion free panes: after that the pressurization had to be rechecked: 75 B-29s in total needed new glass.
Split into simpler sentences: Each piece of glass installed in the nose had to be pulled out and replaced with new distortion free panes. After that the pressurization had to be rechecked: 75 B-29s in total needed new glass.