The leaky air spring seems to be a common item at 50K mi. I just changed my fronts a few months ago w/less than 50K on the odo. I used the Arnott rebuilds too and they worked great.

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'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4