Over charging can boil off the water. Either the charging voltage is too high (on American cars over 14.50 volts too much). Another possible is that the charging voltage is OK but when your battery is fully charged the regulator is not stopping it from charging.
My understanding of batterys are that each cell is separate from the other cells; so if you had a leak it would be unlikely that all of the cells would be dry. Also If a cell leaked out several times and you replaced it with water soon there would not be enough acid left in the cell and your batter would be no good.
I could not find the Max voltage in the factory manual but in the Mitchelle manual it said the output should be 13.7 to 14.5 volts.

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84 300D, 82 Volvo 244Gl Diesel