My rear callipers were shot so got service kits - popped the piston out - cleaned it all out good-put new seals in etc

When i go to put the piston locator seal over the piston (which is metal ring covered in rubber)

Now im looking at it, and thinkin - its way too small - so force it over and it snaps-get another and also breaks!! I look at both original ones, also split

Now i guess this holds the fluid pressure behind the piston so is kinda important, is there a special tool or way to get this f***er to fit??????

Also these brakes have always been ****ty - could my bust rings bee at fault?