I don't know who your napa guy is but I looked at NAPAONLINE and they have new mastercylinders, rebuit ones, kits, even master cylinder and booster combos.

I don't know how technical you are but, I always rebuilt them. Masters and wheels. It's simple and easy. With rebuilt you don't know what quality of soft bits you're getting, or the age of the core. Usually in rebuilt they use the  cheapest stuff. And its done by some kid wearing a diaper sitting in the dirt holding it with his bare feet in some fourth world country.

If the booster is plastic replace it. The are really cheaply made and usually fail due to the housing cracking. There are some after market ones made of steel. Try and find one of those. There are also "Universal fit" ones with a couple of extra vacuum fittings that look pretty awful.

The hardest part of removing the Master Cylinder is unhooking it from the brade pedal. Try to find a smalll skinny friend to help with that part.

Also if you have a Male 'vette (stick shift) you might as well rebuild the clutch Master Cylinder as well. With the brake M/C and booster off its right there calling to you. "Don't forget me, I'm old too".

dave,

Thanks everyone for the input.... NAPA and AutoZone have both explained the 89-91 M/C's are on national back order... including OEM, rebuilt and kits. My local NAPA guy checked again yesterday and found there was 1 rebuild kit in Chicago and I grabbed it. Should be here Monday. His best guess is the rebuild kit he found was a return/restock and that's why there was only one.

it runs in spurts dave. you can go and order things and usually get them UNTIL YOU NEED THEM. there have been a couple things i have run across. it took 3 months to get the molding on my front bumper. finally got one out of a junkyard. egr solenoid was the same except they finally had to come out with an aftermarket replacement. on my 75 the proportioning valve went and it took me 7 weeks to get one..

Installed the kit last night and I'm back up and running! All of the parts in the kit fit perfect and the cylinder wall had no signs of wear or scoring. Visually, all of the old components looked fine but I have no way of knowing if the combination piston is okay or not. I was surprised how bad the brake fluid was when I drained it from each of the bleeders. The fluid that came out of each of the calipers was a little milky looking when I initially opened the bleeders, so there was definitely moisture in the system. On top of that, 2 of them squirted out a little junk when I started refilling the system. After bleeding all 4 brakes, I removed all of the bleeders, cleaned them, and bled again. While bleeding the 2nd time around, a little more junk passed through.  I'm now back up and running after 3 weeks of down time. Car stops well and is better than it's ever been during the 6 years I've owned it, but it still isn't as good as I think it should be. I'll bet a beer or 2 there's more junk in the calipers and I see more wear on the rotors/pads with the wheels off than I previously thought. I see a complete brake job in my future.  

great to hear that you are up and running.