After I flush the toilet , as it starts to fill, it makes a long hissing sound that does not stop unless I very gently press the screw on the top left

What screw... and where? I assume under the lid... in which case it is the valve and might have debris holding it open.

What screw...and where? I assume under the lid...in which case it is the valve and might have debris holding it open.

The screw is located top left of where the ballcock is attached to. There are two screws it is the one on the left

The screw is located top left of where the ballcock is attached to. There are two screws it is the one on the left

That's where I thought, just wanted to confirm it... how mechanically skilled are you?

If you are good with tools... and depending on the type of valve... you might be able to turn the water off under the toilet, take it apart at the top and clean it and put it back together... and maybe fix it for a while... but few are designed to come apart. Some you can't take apart... so that assembly likely is due for replacement.

Good news is they aren't very expensive and are fairly easy to replace, usually.

However, give the screw a turn or two first... that should try to have it shut a little sooner... if it works great... if not... back to the above plan.

I like smoothy's second option, just replaace the valve for under $10

I am with Bob and Smothy, if you do want to change the fill valve I suggest to go with the fluidmaster see the image below,Thanks.

John