Brakes do not sound like something you'd want to mess with, even on a classic car for which you might need a part that is no longer common. Those are part of the safety system in your car, and you never know what really happened to salvaged parts before they were removed from their old car. Note here that salvaged merely means salvaged from a vehicle, not necessarily salvaged as in damaged. But still, when is it OK to use a used brake part in your classic car?
Wear vs. Permanent
One thing is for sure: If the part wears down, get a new one. No used brake pads (that should go without saying), no used rotors, no used anything that constantly wears. A permanent part that doesn't wear, though, might be fine if it's in good shape and is not directly responsible for stopping the car. Be aware of corrosion, rusty spots, and weak spots. Have a classic car mechanic inspect the part, too, before you try installing it -- and pay attention if the mechanic says not to. The last thing you need is a weak old part failing as you drive the car.
If your car needs a specific part from an original car because of how the part needs to fit in your car, you may want to look for companies that make reproductions of vintage parts. These are new parts but have the shape you need to make a good fit. These may be a better bet than salvaged parts because you know you're getting new material that hasn't seen a lot of trauma.
Driving and Show vs. Collector's Value
One more consideration is why you have the classic car. If you have it because you want to drive it and show it off, a vintage part actually taken from an old car might not be as crucial. You really just need a part that fits and works, and a reproduction could be the ticket there. If you have the car in order to have a collector's item that appreciates in value, then having an original part becomes more important. However, you still have the issue of wear-prone parts because those can't be old.
Your mechanic may know of companies that make reproductions of vintage parts, like Brake & Equipment Warehouse, and some companies have original parts that were taken from models that were barely used. Check around and see if any meet the quality level you need to keep you and your car safe on the road.