Jeff, pay particular attention to the piston seal lip when you fit the piston into it's bore. You cant get a lip wizard in there like you can with the forward clutch piston & great care has to be taken not to roll the seal lip over the chamfer in the drum. It has to slide down the chamfer & into it's bore. I find the chamfer is a little too shallow, but with a little patience the piston normally drops in fine without too much fuss. The secret is to use lots of assembly grease/Vaseline/ATF to lube the seal lip & the chamfer. Dont be shy with the lube. You'll see what I mean when you get round to fitting the piston.
Cheers. Gaz.