I'm trying to replace the fill valve of a toilet that's missing some parts and I've run into a snag. I bought a Fluidmaster universal fill valve, and the instructions say that the top of the overflow tube should be at least an inch below a certain level on the new fill valve assembly. But the fill valve seems a bit short, and the overflow tube is higher than allowed. Here's a picture:

Inside the tank

It's kind of hard to see because of the angle, but hopefully it's visible how the overflow tube is almost as high as the new fill valve, and certainly higher than the fill valve's "critical level" indicator.

I could cut an inch or two off the top of the tube, but then presumably the water level in the tank will be lower. This is a low-flow toilet so I'm worried that it simply might not flush properly if I let the water level fall. Should I cut the tube anyway and not worry about the lower water level?

If I should just cut the tube, what's the best way to do this? The space inside the tank is really awkward and none of my saws fit. I can get my PVC pipe cutter in there but the tube material is a bendy plastic so it just compresses when the blade hits it.

  • A picture would be helpful. Is the fill valve height adjustable?
    – Tester101
    Feb 8, 2014 at 3:07
  • The float, and that screw between the fill lever and the float should adjust the fill level, not the height of the overflow tube. The height of the overflow tube is a safety for if the valve fails open, then the water rises up over the float and goes down the overflow tube without overflowing the tanks and pouring onto the floor.
    – Dave X
    Sep 5, 2023 at 18:24

6 Answers 6


I think @Tester101 has asked the pertinent question: Is the fill valve height adjustable? It is. You need to adjust its height:

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  • D'oh! Somehow I managed to read that part of the instructions without figuring out that the whole valve assembly was adjustable. That's done it; thanks guys!
    – iLikeDirt
    Feb 8, 2014 at 5:22

This is an old issue, but it was very handy in my Dec 2016 solution. I believe a key item in my resolution not covered here is why is the CL (critical level) where it is at. Raising the top of the fill valve has a purpose, but that is not important here. In my case, the flapper handle lever creates it's own "CL", meaning, if water rises above that, it will become my tanks overfill, right onto the floor, and wherever gravity wishes to take the stream of continuous water flow. So, I needed to shorten my overfill tube to below the handles ceramic hole, so the overfill tube could do it's actual function. There are reasons for the fill valve's height, but judge your critical level on it's own merit.

It's been at the wrong height for a long time whenever someone swapped out likely a metal overfill tube. Someone else's shortcut creates someone else's mess.

By the way, I found a 30 year old little toy hacksaw that was perfect for cutting the plastic overfill tube without removing it.


The overflow tube is supposed to be higher than the normal level

And it has nothing to do with the normal operating levels in the tank.

It's like the "emergency spillway" on an earthen dam. It should never be used, and it should sit well above the normal operating range.

The overflow tube is the last defense against the tank completely overflowing, either over the top or out the flush-lever hole. Hold down the float so the tank overfills. The water should climb to the top of the overflow pipe and go down it. If it goes out the flush lever instead, that is the only time one would ever cut down an overflow pipe, and only that much.

Again, the overflow pipe should sit well above the water surface.

If water sits at the top of the overflow pipe, that indicates a defective fill valve that won't shut off entirely. The moogie on your fill valve suggests just that has been going on for quite some time. Oh wait, you just replaced the fill valve... Well, the new one shouldn't come this high.

See also the legend on the back of the tank wall that says where fill level should be. That seems to agree with the instructions for the fill valve.

  • It's not supposed to be higher than the flush-lever hole, otherwise you can leak onto the floor.
    – Jason S
    Jul 31, 2019 at 1:09

Had the same problem - the new Fluidmaster Duo-Flush has been redesigned. the clip for the fill hose sticks up above the tank. Looks like the fill pipe that was installed with the last Duo-Flush was too tall. I didn't want to remove the whole tank and install a new overflow pipe and flush valve seat so I had to take 1 1/2 inches off the overflow while it was still in the tank. I found that a hacksaw blade from a reciprocating saw used by hand was the best option to shorten the pipe. I had to file the cut end smooth, but it worked pretty good. The hardest part was cleaning the plastic "sawdust" from the tank. Next time I think I would put a plastic bag or newspaper in the tank to catch the dust. Live and learn. It's back up and working great. A five minute job turned into an hour and a half, but since it's Memorial Day weekend it really couldn't wait...


I have the same situation because the instructions say the top of the new fill valve should be 3" above the overflow tube and in my tank that would place the top of the fill valve above the top of the tank so I plan to take 1" off the overflow tube


I cut an approximate 6 inch section from a 1 liter plastic bottle, then cut height-wise to make an approx. 6 inch x 10 inch rectangle. I rolled it up and tucked it into the overflow tube. No glue needed! It works great!

  • 4
    The OP has stated that the overflow tube is higher... your solution would make it even higher.
    – JACK
    Aug 8, 2021 at 19:58

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