Rear caliper rebuild HELP!

I tried to put my calipers back together yesterday after cleaning and painting them etc, but now have a problem. I have a seal left and I don't know where it goes! I'm sure I never had one of these on when I took them apart plus I can't find any info on the net. I really need to get them in tonight as the car failed the mot on Saturday due to the rear caliper!

It's the second smallest o ring in the kit. And it's kind of flat too.

Also, when I move the handbrake lever without the piston in, the spindle moves out about 5mm. But when I do it after the piston is wound in, it only pushes the piston out the first time, then hardly moves at all after. Is that normal, does it need to be full of brake fluid and pressurised to work properly??