I agree with the above again but while your there mate why don't you take the caliper off (leave it connected but 'dismount it') and give it a good clean, pump the pistons out A LITTLE BIT clean them up before pushing back into the caliper that on it's own could help you out here.
Regular brake maintenance, deffo worth the extra time and will help you spot any problems like seals coming out or scored pistons before it's too late, and a good old bleed for good measure.