I ma having a similiar problem. I already tried using muratic acid,
and replacing the float valve, and I've also tried the bucket test.
The acid cleaned out all the rust stains really good, but other than
that, no effect. Water pressure and drains all work good in the sink
and tub in the same bathroom. Any ideas? Seems like I've rules out
everything! The only thing I can think of that's left to do is to use
my 25' snake from the sink and make sure that the main from the
bathroom to the basement is not having any major problems. Any other
ideas? I suppose I could get a toilet auger, but I fail to see the
point of doing that when it passes the bucket test. It's not like I
got kids dropping stuff down the toilet either.