In my case it turns out that little pieces of rust from the metal widgets in there were causing tiny leaks in the flapper. I'd suck out all the water, totally flush it out and make sure it's pristine in there then refill and test. Annoying! And wasteful!

That's where your leak is. Any one can do it. Turn off the water, flush the toilet, reach down in the
tank and detach the flapper valve from the stand pipe by pulling on the little tabs on each side, up and to the side. Take it to any Hardware store and get a replacement.

One of the things the previous homeowners told me was that the toilet would fill constantly from time to time. I replaced the flap valve and shortened the chain one link and haven't had the problem, at all.

The constantly filling toilet is usually an easy fix. Most of them can be fixed with a new flap valve and replacing the filler assembly itself is usually pretty easy.

And polish the seat that it seals against. If there is a tiny nick in the seat then you will have to replace the seat as well.

Sometimes a kind of slimy fungus builds up on the underside of the flapper. It will allow a tiny amount of water to pass. If it feels slimy clean it with CLR cleaner and clean the flapper seat as well. Do not use any abrasive cleaners, not even Soft Scrub. They will leave tiny nicks in the seat that will pass water.

If your best efforts don't resolve it, call a plumber or replace the toilet with a low volume flush model. Adjusting the water level did not save much water; like a proverbial drop in a bucket. 45 years is a full life for a toilet; it doesn't owe you anything and you are wasting a lot of water every time you flush it, even after adjusting the level.

it may be even a new flapper cannot seal around the flush valve seat because the flush valve seat is worn and uneven. When for example a toilet is rebuilt all parts in the tank and tank to bowl are replaced.

There is however a repair kit you can buy at HD or lowe's that slips on to the top of the flush valve seat thus allowing the flapper to seat itself correctly. That or rebuild the tank by replacing all inner tank parts and tank to bowl parts. Since you already replaced the fill valve ,that part isn't necessary. The fill tube and the the seat the flapper seals off are connected or one piece.

If you have never rebuilt that tank and tank to bowl parts meaning replaced all parts,, it just may be about time you do that.

Or get the repair kit for the flush valve seat, about $ 8.00

Good luck.

My son-in-law will be coming within the next couple of weeks to tend to this. Meanwhile, there is always downstairs. What a pain in the pa-toot. When these old bones were about 20 years younger, I would have done this myself. Now, if I can't get family to help, it is a big bill out of a small income.

How life changes before we even think much about it.

I didn't feel like rebuilding it and didn't want to buy a new one either.

So I went with a dule flush velve ,haven't had a problem since.

Turn off the water to the toilet, flush, then place a cloth through the flap valve a few inches down towards the bowl, leaving the rest of the cloth in the water remaining in the tank. Let it sit for 15-20 minutes and you will have a tank completely empty of water.

This is a capillary siphon. The water drawn into the cloth and gravity causes the water in the end tucked down the flap valve to drip into the bowl, pulling more water into the cloth. Make sure the cloth doesn't fall all the way through.