Tip Seals keep failing

Is there a reason why the tip seals in a VArian SH-110 Scroll pump would keep failing? Most recently, the tip seals failed after about 200 hours of operation. This is the third set of tip seals on a rebuilt pump. The place where we bought the rebuilt pump keeps saying the tip seals need to be replace. The pump has been behaving the same way since we purchased it (won't hold foreline pressure in a turbopumped system). Thanks!

Equipment: Varian SH-110 Scroll Pump