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I'd do a visual inspection of the reservoir if it is rusty it either has had water inside from condensation or bad storage. Most replacement master cylinders, MCs, are pressure tested at the factory. I've found that most are wetted with brake fluid which is used as an assembly lube to protect the seals so look for evidence of that.
Add a little brake fluid to the reservoir and pump it a couple of times with the cover in place until you get fluid squirting out the port. Now block off the exit port on the master with a 7/16-24 bolt wrapped with a turn or two of Teflon tape. Thread engagement must stay clear of the female MC raised cone. This size assumes you have the stock 1/4" brake lines. You could also use a short stub of crimped off brake line. Add some new clean brake fluid to the reservoir and try to push in on the plunger. It shouldn't move but a tiny amount.
If you had a dual bowl MC, you would have to bench bleed to get rid of any air in the MC bore. This is not required for a single pot MC. Once bench bled, you can check it as above for a single pot MC.