In the UK we are new to flapper valves and few plumbers know much about
them. I had a customer with a similar problem to yours with an old
cystern that looks rather like the one in your photos. My solution was
the silicone process but somewhat differently applied to the other
reply on this subject. Assuming the flapper itself is removeable I
applied builders silicone sealer (silicone that solidifies after an
hour or so) to the underside of the flapper and then (this is the
sneaky bit) rested the flapper on a piece of lightly oiled plastic
film. When dry pull off the film and you have a smooth and soft base
to your flapper.
If you can get to the top of the brass ring (seat) you could also dress
the ring VERY LIGHTLY with an abrasive paper, something like a wet/dry
silicone carbide paper used in car bodyshops, but only rub around the
ring (clockwise or anticlockwise) and not across the seat. You will
probably find that the seat itself has a slight grove across it which
will need to removed by rubbing the rest of the ring down to match.
Panistaking job but when done will work like new.
Dont throw out such a lovely old toilet. Items like this will soon be
worth money if the UK market is anything to go by. Old cast iron baths
with feet are now fetching in the order of 1000 uk pounds regardless of
condition. You might soon be sitting on a fortune!!

T. T. wrote:
Hi all, we have a 1935 American Standard antique toilet which might
have a faulty seal beneath the flush valve (for pictures please see
http://www.picturetrail.com/toilet2006/). The thing is that it is an
old toilet with brass (I guess) seal, instead of a plastic one, and I'm
not sure whether the size of the seal is different from the generic
seals that you could get from, say, fluidmaster. I changed the flapper,
but it still can not create a tight closure against the flush value.
Water still leaks down the drain (very slowly). For this kind of
antique toilet, could I still use say the Fluidmaster (fluidmaster.com)
Complete Repair Kit (model 400AK) or a flush valve to repair it?? If
not, what other options do I have to repair it?? Also, I think this is
a 3.5GL type of Toilet which is not conserving water, if I change the
Fluidmaster Complete Repair Kit, does it still flush with 3.5GL, or it
would become 1.6GL?? Thanks!