Like stated in the summary, my toilet tank won't fill up unless I turn off the water valve (beneath the tank) and then turn it back on again. I don't have the ballcock type assembly. I have the 400A type of fill valve.

Let me add that right before this started happening, I turned off the main water supply to the house, turned on all the faucets and flushed the toilet until water stopped running from the faucets and toilet. Then I turned off all the faucets and slowly turned back on the main water supply. (I did that because another site said that that would fix knocking pipes.) Anyway, the next time the toilet was used (about 1/2 hour later), the tank did not fill. But if I turn off the water valve under the tank and then turn it back on, the toilet will fill. But I have to do that every time the toilet is flushed.

You have a FluidMaster ballcock,(see image) Try this. Go down to the hardware store and purchase a FluidMaster # 242 replacement seal, (see image). Shut the water off to the toilet and twist the top ofc the ballcock and replace the seal. Before you replace the top place a pan over the open ballcock and turn the water on for a minute to flush out the supply. Good luck, Tom

Like stated in the summary, my toilet tank won't fill up unless I turn off the water valve (beneath the tank) and then turn it back on again. I don't have the ballcock type assembly. I have the 400A type of fill valve.

Let me add that right before this started happening, I turned off the main water supply to the house, turned on all the faucets and flushed the toilet until water stopped running from the faucets and toilet. Then I turned off all the faucets and slowly turned back on the main water supply. (I did that because another site said that that would fix knocking pipes.) Anyway, the next time the toilet was used (about 1/2 hour later), the tank did not fill. But if I turn off the water valve under the tank and then turn it back on, the toilet will fill. But I have to do that every time the toilet is flushed.
They didn't have the replacement seal so I just bought the whole thing. I'm going to replace it and let you know how it goes. I still have one question though. Why would turning off and on the water supply make it work for one flush? And now this morning, I have to turn off the water, run the water in the bathtub, and then turn back on the water supply before it'll work. I'm just curious.

I still have one question though. Why would turning off and on the water supply make it work for one flush? And now this morning, I have to turn off the water, run the water in the bathtub, and then turn back on the water supply before it'll work. I'm just curious
And I'm still in the dark! I wish I were there to check it out. Keep me in the loop on this and let me know how you make out. Are you replacing with another FluidMaster? Regards, Tom

I received a speedy, accurate answer to my problem.

Well, I replaced the fill valve with another Fluidmaster 400A and it works fine now. I guess it will remain a mystery as to why it worked for one flush after going through the routine I described. Thank you so much for all your help.

Thanks for posting this as I am having the same proble. I have to turn off and then on the water supply under the toilet before the tank will fill up. If I just leave it after flushing, it will fill up in time, but I will try replacing the seal.