I replaced the fill valve and the red flush valve seal on my #160 model, but I hear tank water being replaced every so often, So I guess there is still a leak. The black ring I see at the end of the flush value is crumbling, so I would guess I need to replace the whole flush valve. I am thinking I have to remove the whole tank to do that, right? Or is there a simple way? Do I need a tank to bowl seal too? Are all toilet tanks this complicated?

I would replace the toilet with a decent unit before I ever spent money on a mansfield toilet. You can get a high quality toilet such as a Toto Drake for about the same price as a mansfield and replacement parts to change out the junk installed at the factory.

our sponsor has a good supply of mansfield parts.
you will need to remove the tank to change the flush valve. i would also replace the tank to bowl bolts and washers.
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the red washer is replaceable. look for an online video on how to do this.

Give the red seal replacement a second look. It is imperative that it is exactly positioned in the groove for it to seal correctly.