Lift the lid of your toilet tank and you will see a small group of parts doing a simple job. When the tank empties after a flush, a float drops, the fill valve opens, and water comes in. When the tank is full again, the float rises and the valve closes.
If the float misjudges that moment, you get a toilet that runs, refills by itself or hisses all night. Many of these problems come down to a float that needs an adjustment, a replacement or just a rinse.
Which type of float do you have?
The float comes in a few designs, and the fix depends on which one is in your tank. Look at the part next to the tall fill valve on one side of the tank.
Ball float on an arm
The old style, with a plastic or metal ball at the end of a rod. The rod bends or an adjusting screw changes the water level. It is simple, and it fails when the ball takes on water or the rod corrodes.
Cup float on the fill valve
A plastic cup slides up and down the fill valve shaft. A clip or screw sets its height. It is the most common design in homes today and the easiest to adjust.
Float arm with a pull rod
A thin arm moves a rod connected to the valve body. These show up on many fill valves of mixed age, and the rod can be bent by accident when someone moves the tank lid.
Pressure sensing valve
Some modern valves use no visible float at all. They sense tank water pressure instead, and the fix is replacement rather than adjustment.
Signs the float is the problem
A float fault usually shows up in one of a few ways. You may hear the fill valve hissing long after the flush, see water trickling down the overflow tube in the center of the tank, or find the tank water level so high it touches the top of that tube.
A low water level is the opposite problem. The bowl may flush weakly, because the tank is not filling enough. In both cases, the float is telling the valve the wrong thing, either because it is set badly or because something is binding.
Step by step: adjust the float
Shut off the water and look
Close the small valve on the wall or floor behind the toilet by turning it clockwise. Remove the lid and set it somewhere safe. Flush once to empty the tank and watch where the float sits.
Find the water level mark
Many fill valves and tanks show a line marking the correct water level, usually about an inch below the top of the overflow tube. If there is no mark, aim for a level just under the tube opening.
Adjust the float
On a cup float, squeeze the clip and slide the cup down to lower the level or up to raise it. On a ball float, turn the adjustment screw or bend the rod gently. Small changes go a long way.
Turn the water back on and test
Open the valve slowly, let the tank refill, and see where the water stops. Flush again and check that the fill valve shuts off cleanly. Repeat until the level sits just below the overflow tube.
Check for leftover problems
Listen for hissing and look at the overflow tube. If water still runs, the problem is not just the float. Time to look at the fill valve or the flapper.
Float, fill valve or flapper?
Many people replace the wrong part because the symptoms overlap. A quick test can separate them. Put a few drops of food coloring in the tank, wait without flushing, and look at the bowl. If color shows up in the bowl, the flapper is leaking and the float is not your problem.
If the bowl stays clear but the water keeps running into the overflow tube, the fill valve or float is not shutting off. This table shows the usual cause for each symptom.
| What you notice | Likely cause | Where to start |
|---|---|---|
| Hissing after refill, water at top of overflow tube | Float set too high or valve not closing | Lower the float, then check the valve |
| Tank refills every so often with no flush | Flapper leaking | Food coloring test, replace the flapper |
| Weak flush, tank only half full | Float set too low | Raise the float to the water line mark |
| Float scrapes the tank wall | Float cup or arm out of position | Reposition, check for a bent rod |
| Water never stops even after adjusting | Worn fill valve | Replace the fill valve assembly |
Replacing the fill valve and float
When adjusting does not help, the float and fill valve are usually replaced together as one assembly. The old valve unscrews from the bottom of the tank after the supply line is disconnected, and the new one goes in the same hole. It is a manageable job if you are patient with the nuts and the water line.
Set the height of the new valve before you tighten it so the top sits where the manufacturer says, and connect the supply line without overtightening plastic fittings. Turn the water on slowly and test the shutoff a couple of times. If the shutoff valve behind the toilet leaks or will not close fully, stop there, because that is a separate repair.
Millbrae water pressure and your float
Millbrae runs its own water system from San Francisco Regional Water System supply, and the city's system is divided into 11 pressure zones. That means water pressure at your house can differ from a neighbor's across town. High pressure pushes hard against a fill valve seat and can make a float valve fail to seal fully, while very low pressure refills the tank slowly and can look like a float fault.
If your toilet and a few other fixtures all show odd symptoms, such as faucet hammer or hissing, pressure may be part of the picture. We can test it at an outdoor hose connection. Millbrae's water is chloraminated, and rubber flappers and fill valve seals do wear out, so replacing them when they go soft is routine care.
The city's Public Works department runs the water service in Millbrae, and a running toilet adds to what is metered at your house. A small float fix is a simple way to protect your bill.
What you can do and when to stop
DIY or what people try
- Bending a float arm until the water level looks right
- Replacing the flapper when the float is the real fault
- Tightening plastic fittings with a wrench until they crack
- Leaving a running toilet for weeks and hoping it clears
Professional approach
- Running a dye test to separate flapper leaks from float faults
- Testing house pressure when several fixtures act up
- Replacing the fill valve and supply line as a matched set
- Checking the shutoff valve and bowl seal while we are there
A running toilet wastes water around the clock
If the float adjustment did not stop it, call the 24/7 line. The cause is usually easy to spot once the tank lid is off.
When to call a plumber
Call if the shutoff valve behind the toilet is stuck, corroded or leaks when you turn it. Call if the tank cracked, if water is coming from the base of the toilet, or if you have tried a float adjustment and a new fill valve with no change.
Our toilet repair and installation service covers floats, fill valves, flappers and full toilet replacements. Because a bad shutoff valve and a bad faucet often show up in the same bathroom, our faucet repair crew can look at both in one visit. We serve Millbrae and nearby areas such as Burlingame, where our Burlingame plumbing page has the details, and the service area page lists the rest.
Toilet float questions
Look for a tank that never stops filling, water running into the overflow tube, hissing after a flush, or a float that scrapes the tank. If a dye test shows color in the bowl, the flapper is the culprit instead.
On an old ball float arm, a gentle bend often changes the water level. Bend slowly and in small steps, because brass and plastic can snap. On a cup float, use the adjustment clip instead of bending anything.
Usually about an inch below the top of the overflow tube, or at the line marked inside the tank. Too high wastes water through the tube, and too low weakens the flush.
Many homeowners can do it with patience. You shut off the water, drain the tank, disconnect the supply line, swap the valve and reconnect. Corroded shutoff valves are the usual snag, and that is when a plumber helps.
Usually a second fault is hiding behind the first. A worn flapper, a mineral crust on the flush valve seat or high water pressure can all keep a toilet running. A dye test will show which one it is.
Yes. Millbrae's Public Works department provides water service, and water that leaks past a toilet float still passes through your meter. A steady trickle can add up over a billing period, so fix it promptly.
Sources and standards: The California Plumbing Code (based on the Uniform Plumbing Code). The City of Millbrae 2025 Water Quality Report.