Toilets, Sinks, Showers, Tubs and Disposals - Strange Toilet Refill Issue

I noticed a very strange water refill issue in the main level bathroom. Not having this issue with any other bathroom. When someone has the faucet for the bathtub turned on located immediately above the toilet on the main level will NOT refill until the bathtub water is turned off. I noticed that this is not a problem if the I turn the diverter to the shower on only when the water is coming out the bathtub spout. As soon as the bathtub faucet is turned off, the toilet immediately begins to refill. It otherwise refills normally. I tried turning on other faucets in the house to see if it causes a similar issue but none of them causes the same issue. The toilet uses a fluidmaster refill.

We have the exact same issue. It only happens when the upstairs tub is on. They are on the same suply line. I dont know how the water passes by the toilet to get to the tub without filling it. I have lifted the tank to see what happens and only a very small amouint of water enters the tank.

Water doesn't flow to one fixture first, then to another one. It flows to the one with the least resistance. The toilet probably has a 3/8" maximum pipe leading to it, while the tub has a 1/2" piping. In addition, it is entirely possible the initial plumbing was all 1/2" rather than 3/4" to the turn up point of the fixtures, then 1/2", which will account to all the water going to the 1/2" pipe first.

Is this problem the same if you -just- run cold bathtub water or if you run -just- hot bathtub water? or does one or the other scenario make the toilet run slower? The answer might give clues to what -1/2 inch old galvanized piping?- pipe might be getting clogged with rust?

It seems to have the same issue whether the tub faucet is on full cold or on full hot. The strange thing is that the toilet will refill if you divert from the tub faucet to the shower. With the shower on it refills a little slower then when compared to when the water is off completely but water does come into the tank. Also, not sure if it makes a difference but that tub has one of those single MOEN pull-out handles.

You will probably find out that if you do a timed water volume test between the flow coming out your tub spout, and then compare to another timed test of the water coming out of the shower head, that more water comes out of the tub spout. And that would explain why the toilet fills more normal when the shower is on.

If your slow to fill toilet problem is the same with hot tub and cold tub, then I think your problem may lie in the cold water line before ito gets to your water heater. If I could see your entire house plumbing I, or someone else here, might be able to figure this out. But since I can't, the only way I could tell is to either simply start making various plumbing drawings whose layout could replicate your problem, or have you provide us with a layout as to where all the branch lines are in relation to each other after you begin with your meter and say what comes first, what comes next and what sizes every pipe are.

It's possible that you have a problem that is curable without ripping into walls.