Plumbing in the Home/Toilet not filling
I live in a 1924 house with what looks like the original water supply lines. The house is also a duplex with one flat over the other. The toilet in the lower flat is not filling with water. All other plumbing fixtures are working just fine. The toilet originally all of a sudden stopped working. We took it all apart. We replaced the flush valve and the hose. This fixed the problem in that the water would flow in, but the flush was very weak and the toilet would run. Then today the toilet stopped filling again. We disconnected the supply hose from the toilet and no water was coming out. We disconnected the hose from the valve at the wall and water comes out of the wall. Other information, when we turn the valve on there is some water hammer effect that happens in the pipes in the basement. We found that the toilet would work if the valve was only half open. Since this is our only toilet we did that. Then later tonight it stopped filling again
now there is this constant water flowing and drip sound coming from the toilet. I'm not sure if his is relevant, but there is a smell that seems to always come from the toilet. We clean it and it just comes back. The toilet is an American Standard one piece 1.18 gpf toilet with a small tank. Thank you!
If indeed these pipes are the original pipes installed in the building (possible but seems unlikely) it is likely they are galvanized steel and possibly quite corroded on the inside.
You mentioned that there is some occasional water hammer. This is typically caused by a rapidly opening and closing valve somewhere in the system that causes the water flow to pulse. This transfers a lot of energy into the piping system causing it to rattle or knock. It is possible that this is being caused by a fluttering of the internal diaphragm of the toilet fill valve.
The reduced water flow might be caused by particles of debris and rust shaking loose from the pipes and traveling up the line until they get stuck somewhere, thus creating a blockage which slows or stops the water flow. Toilet fill valve diaphragms will often flutter often on if the water flow is quite low, not enough to keep it fully open.
This type of problem is rather difficult to diagnose long-distance. It seems like you have been doing some troubleshooting and have determined that, while the water delivery from the shut off valve under the toilet seems to be okay, the toilet supply line from the valve to the base of the fill valve may be blocked. So that's the first thing that I would do would be to replace that supply line or attempt to clear it out. If this continues to happen, it is an indication that the water pipes in the building are starting to fail and may have to be replaced.