Thats a pretty common gixxer problem, mate. The pistons will be dirty, and not freely moving in and out of the calipers. You need to clean them thoroughly, one caliper at a time (leave the other one on the disc), by removing the pads, then pumping the pistons out, cleaning all down and around them, then pushing them back in to the caliper and repeat and repeat. Try not to actually pop the pistons all the way out, our you'll get brake fluid everywhere and let air into the system.

Whilst it's not fantastic for the seals, a good spray with brake cleaner is usually the only way to get them properly clean. Doing them with something more seal-friendly (Simple Green, or just brake fluid etc) often just won't do the job properly.