I've tried an Ebay master cylinder for an '82 CB900C but didn't have any luck. I'm convinced, however, that my problem was simply the result of using a master cylinder with a bore that was too small... it was simply for a single piston caliper, not my caliper with 2 pistons. The same master cylinder worked fine on my SOHC CB750 so I wasn't out anything for trying it, and was actually happy to have found a replacement for that bike.
Long story short, I'd suggest that you search Ebay for a CB750C master cylinder, then read closely to see whether or not it has the same size piston as yours, and that it specifically states that it's for a 2-piston caliper or 2 calipers, or whatever it is that your bike has. Asking the seller if the one you're looking at should work on your bike would be good as well, especially if you end up finding that it doesn't work and you want to return it.