Ford Mondeo 1 break issues?

I have a 2001 Ford Mondeo (B5Y) with a brake problem. The back left brake was wearing down so I replaced the rear brake shoes and Disc's. Within a 3 month period the grinding was back on the rear left brake. Checked the saddle and found that the rubber seal was torn so replaced the saddle, shoes and discs again. 2 months later the grinding was back again on the rear left brake (Outer pad only, inside was almost like new). Replaced shoes and disc's again and checked after 2000km, the outer pad was worn down again. Replaced rear brake lines but am still hearing rubbing from back left brake. Any ideas????
Thanks, Chris