Found My Mom's Pressure Cooker

My mom gave me her pressure cooker several years ago, but I've never used it. I dug it out of a very high cupboard and decided to learn how to use it. The lid was on tightly ... so tightly that my husband had a hell of a time removing it. It appears that either mineral deposits or perhaps microscopic bits of food residue had formed an almost hermetic seal. These deposits were all over the interior of the pot and the seal. I tossed in some kosher salt and began giving it a hearty scrub. The rinse water was dark grey. I did another round or two of salt, then a round with baking soda and vinegar. The pot feels smooth inside, but there are some small grey spots and the seal still has light colored spots (possibly mineral deposits) all over it.

Should I toss it? I was hoping that this would be an oldie but a goodie, but now I'm afraid that the pot has been severely compromised. If only she'd not locked the lid on all those years ago. It has probably been 20+ years since it was last used.

The handle says Presto Supreme Aluminum Industries Limited. It was made in Canada. Is it safe to use, health-wise? I can get a new sealing ring online, but should I bother?