Don’t change your toilet tank fill valve just yet
Most of the posts I have seen about this automatically assume that you have to replace your toilet tank fill valve. I have recently learned that this is not necessarily true. If you have a toilet tank fill valve like the one seen on the left (http://www.fluidmaster.com/toilet-fill-valve.html) then you can actually just twist the top off and clean it. The hissing occurs when gunk inevitably builds up in the gasket hidden under the top.
If you clean up the gunk under the top and replace it – the hissing stops. This is much easier to at least try before replacing your whole valve – which can definitely be a pain in the ass to do.
All of the instructions you need are here: http://www.fluidmaster.com/new-toilet-fill-valve-won-t-turn-off-cant-get-top-off.html
There are even visual instructions on YouTube right here from FluidMaster: https://www.youtube.com/watch?v=AabLItDN-qk
Cleaning the top
This is my version of the instructions:
- Turn off the water supply
- Place your right hand comfortably on the valve stem below the floater.
- Place your left hand on the top
- Twist the top left (lefty-loosey) while holding the valve stem with a stiff grip
- Pry out the gasket under the top with your fingernail, don’t use a knife
- Clean the gasket, use some hand soap – don’t go crazy
- Take a cup or something that fits on top of the valve stem (no top on at this point)
- Turn on the water supply – this should flush out all the crap that is in the line
- Turn off the water supply after you think it is clean enough
- Put the gasket back
- Twist the top back on (righty-tighty) using the opposite of the method described initially
- Turn on the water supply
- Flush the toilet to test
- If the hissing persists then you should replace your toilet tank fill valve