Pre-1980s: the high-volume era. Using anywhere from around 3.5 to 7 gallons per flush. What's good: they flush powerfully. Volume solves a lot. What's not: the water use is enormous by modern standards, and internal parts on units this old are often obsolete or only available as generic substitutes. Tank and bowl castings can develop hairline cracks with age. The verdict: basic repairs are usually still possible and cheap, but this is the generation where replacement pays for itself in water alone — often faster than people expect if the toilet is used heavily.