How can you tell if the mechanical advance is bad on a distributor? The haynes manual says turn the rotor clockwise, and the springs will pull it back to the original position. Mine doesn't do that. When using a timing light, the timing advances with the vacuum hoses attached, and revving the engine(Timing set at 5BTDC) When i take off and clamp of the hoses, the distributor shows no advancing while revving the engine.

Time to take it apart and clean/rebuild it? 1987 SR5

Yes. Just a few drops of penetrating oil(wd40 is NOT penetrating oil) is usually enough to free it up, but dis-assembly & polishing away the rust/corrosion is necessary for the repair to last.
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Also, forget the timing marks & light BS, go with what runs best - idles smooth & fast, only pings when abused. Checking the advance is the only thing a light is good for IMO.