Unbolt the caliper from the backing plate take the pads out, then CAREFULLY pull the brake lever so as to push the caliper pistons out a bit ( around 5mm ) if there is a line of rust or crap on the pistons clean off with clean brake fluid ( don't use anything else ) you can use a cotton bud or something similar , then push the pistons back in by hand , clean the pins and lightly copper slip them then put the pads back in , reassemble then replace the fluid ( if more than a couple of years old ) with new fluid .
Hopefully that should sort the problem out
