That is not normal. The wagon should hold it's rear height without sagging even if left for weeks. To the best of my knowledege the internal leaking doesn't do any real damage (other than maybe to make the pump work harder) but it's not a hard fix and the car sure looks better when the it's not sagging.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold