I have just got a bike with K18s and when I came to replace the front pads for the first time [after a disappointingly short two months] discovered that one piston was coming out more than the other. I remember many happy hours massaging XT pistons with transmission fluid to try and equalise piston action but thought I was past this. Is this a common problem and is there a particular solution or is it tooth brush and brake fluid again?

yep. I have done this with both ends of my formulas though i cleaned exposed piston and then applies silicon grease to the edge of the piston with the pointy end of a ziptie.

Your brake pads last 2 months?

*jealous*