I would think the master cylinder would build a decent amount of pressure to the pedal, giving it a good bit of feel when you press it down, but that spring would add a good bit of resistance too, I would check both. Spring is easy to check, master cylinder is harder. I always check the easy things first
Also I ReRead your initial post, did not catch that you put in rear disks. I have read on this forum that depending on which calipers you use you might want to swap over to a 15/16ths master cylinder to regain pedal feel due to the increase in required fluid to actuate them. You can pull that from the 1979-1981 ZX's I believe (the ones with the dual reservoirs). It is pretty much a bolt on part, just need to adjust the brake lines and I think also (cant remember the name of the part) the threaded rod that goes between the master and booster(or pedal and booster) I would look at the brake sticky to be sure.
Also It very well could be a bleeding issue. If it were me I would check for the spring and if its there switch over to the 15/16ths master cylinder(if necessary) and then bleed the crap out of it. The master cylinder would probably be an easy junk yard find, but I would buy the rebuild kit (about 8-10$ at your auto store)
You very well could use a rebuild on your current master cylinder if you keep it being that you have the white oxidation and whatnot inside of it.
I'm gonna add I'm no professional with Z's as most of my Z knowledge is from browsing this forum. I haven't been able to work on my Z in a while.
Maybe one of the more experienced could chime in? I would love to have somebody say I'm right (or wrong, I like learning new things but I would hate to find out I am wrong after you go and follow my advice.)
Lol man I just wrote a nice long one huh? try not to fall asleep readin all of it.