I wouldn't worry about the rust on the rotors. The problem you may have though is the calipers may leak because of rust inside the piston bores. Fortunately there are suppliers of stainless steel line calipers that takes care of this problem. 

1973 L-82 4 spd

I think I would sacrifice the old pads cleaning up the rust off the rotors and then replace the pads and flush the whole system with new Dot 4 or 5.1 fluid.  That is assuming you don't have to rebuild/replace the calipers.

Greetings, My opinion is that this is gonna pull your hair out. ANY time you open the system on these cars you'll have a can of worms. They are solid mounted, and are a poor system. The pucks move, not the caliper. If they don't leak now, they will later if the bores are pitted or rusty. Check runout of rotors. If they are still riveted, and not undersize, don't take them off. Runout of more than 5 thousandths is gonna be a problem. Factory tolerance is less. DO get stainless lined calipers, and replace ALL soft lines. Make sure the hard lines are also in good shape. Check for vacuum leaks if power. In good shape they'll scare ya, but they'll stop ya. Do a good job the FIRST time, and you won't have to do it again. 

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Doesn't the '75 use DOT 3 fluid?

Duntov Motors disagrees:

"The floating shoe feature is a safety, as well as performance advantage. Only 5 thousandths of an inch retraction (of the pad from the surface of the disk) equals an inch of pedal travel and 410 PSI of pedal pressure reserve. The C2-C3 brake system was designed to rely on that 410 PSI pedal reserve, which is not available unless the pad is floating on the surface of the disk.

Bottom Line: If your Corvette is a static display show car, O-ring calipers are a good option. If you drive your Corvette, stick with Duntov’s design. If you drive your car only once a month, just depress the brake pedal every couple of weeks, and your lip seals should be leak free for at least a decade."

More here

Personally for daily drivers, I prefer DOT5 because it is hygroscopic and a plus feature is that it will not strip your paint if you accidentally spill it on it. 

1973 L-82 4 spd

I did replace the calipers and lines years ago with stainless steel ones from Vette Brakes & Products. The calipers & lines are not leaking now. However, I didn’t engage the brakes when I started her up. The brake fluid probably had 10 miles of use before I stopped driving it. Does brake fluid naturally degrade over time just sitting in the master cylinder and lines?

Moisture tends to get into brake systems -- DOT3 is alcohol based and absorbs the moisture, most of the others will not absorb it, so the moisture will lay in the system; neither option is ideal.

Brake fluid can also dry out over a long period of time, leaving a residue in the system.

Your best option, as someone else suggested is to purge out the old fluid replacing it with fresh.

Exactly Jim.

On any of my restorations, whether it be auto, truck, or bike, I go through the brake system first.

Steve

As has been said, I'd use the old pads to clean the rotors of rust and then replace them.  I wouldn't mind betting that it will not take much to clean the rotors.

But I would change the brake fluid, getting as much (or all if possible) of the old stuff out.  

I have never got involved in building callipers - my wife and I have done pretty much everything on our '78 in the 34 years we've owned it, but I let professionals build such a vital part!   But we have changed callipers.  I have had a recent discussion with a friend about the merits of O-rings over the lip seal design and I'm afraid I do not agree with Duntov's take on the O-rings.   

Our car gets driven at least once a month, even in the winter on nice days, usually 100 miles or so to "warm everything through", and in "the season" we'll also do various trips both locally and long distance (2000+ miles). When we had the old lip seal callipers it was necessary to bleed the brakes each year to maintain a firm pedal.    About 15 years ago I eventually replaced all 4 callipers with O-ring examples and that need was eliminated, the pedal staying firm for years.   I have now reverted to a routine bleed each year, just to keep the fluid fresh, but no air enters as it did with the lip seals.

I have stayed with DOT4 fluid, which is compatible with what the came with and I see no advantage in going to DOT5.1

What I would add is that clearly, braking performance on a modern cars can be incredible, but our C3s are now at least 42 years old and compared to other cars of the era, the brakes are, or should be, bloomin' good!