My understanding is that there's a teeny tiny hole that allows fluid to return where it belongs when you let go of the brakes. If that hole gets clogged (and it happens a lot on these old bikes) the brake will stick. After you ride around for a while, the brake will stop opening up when you let off of them, and it will act as if you have the brakes half-applied all the time. (this happened to the PO of my bike, who just removed the rear brake altogether, rather than try to fix it.)
When I bought the bike (a 76 RD400), I started with a really clean-looking rear brake assembly, and it worked great for about a year, but it's getting to the point where the rear brake is hot as hell at the end of a leisurely ride, and I think it might be time to clear out that little hole again.
So, make sure (once you get everything apart) you clear that hole before you put things back together, and you should be fine.