Try dropping two regular alka seltzer tablets in the toliet, let it sit for 20 minutes, then use the brush. Good luck
You can get a pumice stone in the comet, bar keepers isle in the grocery store. Drain the water from the toilet bowl, wet the stone an rub the stone on the stain. It comes right off. Works on many things in and around the house!
The best thing I have ever used to clean the bowl is a effervesent tablet. They are used to clean dentures, which are porcelain and do a great job on the bowl. Just drop it in and let it soak awhile. Brush some of the foaming soap over the marks and wait. Good luck
I would like to know the solution because I have the same problem. I've tried almost everything but these black rubber marks remain.