Pry the dust covers off and then use penetrating oil around the pistons. I use air pressure; block one set of pistons with a wood spacer and apply 125psi air to the fluid-in port. If it moves but doesn't come flying out, pound it back in and keep moving it back & forth. Varnish builds up like on lifters and you just need to get by the build-up. Once you have one of the stuck pair out, clean it & put it back a little ways and c-clamp it,then reapply air to move the other one. Scotchbrite the stainless steel pistons and cast iron bores until they're clean, then dig out the seal ring from the bore. P.I and others sell overhaul kits pretty reasonably. Messy, difficult but not expensive job... and you'll be really surprised by stock brake performance, once all the pistons are working!
