How to Test a Toilet Flush Valve (Step-by-Step DIY Guide)

A running toilet wastes up to 200 litres of water per day. Before you call a plumber or pull the tank apart, use this guide to diagnose the problem in under 10 minutes and decide whether you can fix it yourself.

A faulty flush valve shows three telltale signs:

• Ghost flushing – The toilet refills on its own every 15–30 minutes without being used.
• Hissing sound – You hear water trickling into the bowl between flushes.
• Water won’t hold in the tank – The tank level drops gradually, triggering the fill valve to top it up.

If you notice any of these, confirm the diagnosis with the food colouring test.

1. Remove the tank lid and add 5 drops of food colouring to the tank water.
2. Wait 10 minutes. Do not flush.
3. Check the bowl. If coloured water has appeared, your flush valve or flapper is leaking.

This test takes the guesswork out of diagnosis. A positive result means the seal between your tank and bowl has failed.

This is ghost flushing. Water slowly leaks past your flush valve seal into the bowl. Once enough water drains from the tank, the fill valve detects the drop in water level and tops it up, even though nobody flushed.

The cycle repeats every 15–30 minutes depending on how fast the seal is leaking. It’s not dangerous, but it’s wasting water and money until you fix it.

These two components fail in different ways. Misdiagnosis means buying the wrong parts.

If water is pouring into the overflow tube, adjust your float or replace the fill valve. If the tank level drops on its own, your flush valve is the problem. Learn more about the different types of flushing mechanisms.

No, you should not use standard WD-40 on a toilet fill valve because the petroleum distillates can warp the rubber seals and flappers, making your leak worse within weeks.

This advice appears in online forums regularly, but it’s a short-term fix that creates a bigger problem. Petroleum-based lubricants soften rubber components, causing them to lose their shape and seal poorly.

• Silicone grease – Safe for rubber O-rings and seals. Apply a thin coat to restore flexibility without degrading the material.
• White vinegar – Dissolves calcium and mineral buildup without damaging components. Ideal for removing the crusty deposits that cause valves to stick.

If your fill valve is sluggish, noisy, or not shutting off properly, mineral buildup may be the culprit. It can also cause a slow-filling toilet tank. Here’s how to clear it:

1. Turn off the water supply at the tap behind the toilet.
2. Remove the fill valve cap (usually twists or unclips).
3. Place a cup over the valve opening to catch spray.
4. Turn the water on briefly to flush debris through the valve.
5. Replace the cap and test.

This takes two minutes and often resolves fill valve problems without replacement. In hard water areas like Adelaide, Perth, and parts of regional Queensland, sediment buildup is a common cause of valve issues.

Yes, you can replace a flush valve without removing the tank by using a peel-and-stick flush valve repair kit that adheres directly to the existing overflow tube seat.

This matters because tank removal means dealing with rusted bolts, potential cracks, and replacing the spud washer. The overlay method avoids all of that.

These kits fit over your existing flush valve seat. You’re not removing the old valve—you’re installing a new sealing surface on top of it. The result is a fresh, flat surface for your flapper to seal against.

1. Turn off the water supply and flush to empty the tank.
2. Sponge out remaining water from the tank bottom.
3. Clean and dry the existing valve seat thoroughly. Remove any mineral deposits or debris.
4. For peel-and-stick kits: Remove the backing and press the new seat firmly onto the old one. Hold for 30 seconds.
5. For tube extender kits: Cut the old overflow tube to the marked height, then slide the new valve assembly over it.
6. Install the new flapper, reconnect the water supply, and test.

Total time: 20–30 minutes. No heavy lifting. No risk of cracking the tank.

These kits suit standard two-piece toilets with a failed seal but intact overflow tube. If your overflow tube is cracked or the valve body itself is damaged, you’ll need the full replacement method below.

If the quick-fix kit doesn’t suit your situation, or you’d prefer to replace the entire assembly, here’s the complete tank removal process.

• Adjustable wrench or channel-lock pliers
• Flathead and Phillips screwdrivers
• Sponge and bucket
• Replacement flush valve kit (includes new spud washer)
• New tank-to-bowl bolts (recommended if existing bolts show corrosion)
• Towels

1. Shut off and drain. Turn off the supply tap. Flush and hold the handle down to drain as much water as possible. Sponge out the rest.
2. Disconnect the supply line. Unscrew the coupling nut where the supply line meets the tank. Have a towel ready for residual water.
3. Remove the tank bolts. These sit inside the tank and secure it to the bowl. Loosen the nuts from underneath while holding the bolt heads steady with a screwdriver.
4. Lift off the tank. Set it upside down on towels. The old flush valve unscrews from the outside of the tank (large locknut).
5. Install the new valve. Insert the new flush valve, tighten the locknut, and ensure the overflow tube sits about 25mm below the tank’s top edge.
6. Replace the spud washer. This rubber washer seals the tank to the bowl. Always use a new one—reusing the old washer risks leaks.
7. Reinstall the tank. Lower it onto the bowl, insert new bolts, and tighten evenly (alternate sides to keep the tank level). Don’t overtighten—you’ll crack the porcelain.
8. Reconnect and test. Attach the supply line, turn on the water, and check for leaks at all connection points.

Pro Tip: If the tank bolts are rusted or seized, spray them with penetrating oil and wait 15 minutes before attempting removal. WD-40 works fine on metal bolts—it’s only a problem on rubber seals. Badly corroded bolts may need to be cut with a hacksaw.

Not always. Most toilet valve replacements are straightforward DIY jobs, but certain situations make professional help the smarter choice.

DIY It If:

• You have a standard two-piece toilet
• You can comfortably lift 15–20kg (the weight of a full tank)
• Your shut-off valve works properly
• The tank bolts aren’t rusted solid

Estimated cost: $20–$50 for parts from Bunnings or your local hardware store.

Call a Plumber If:

• The tank bolts are corroded beyond removal
• You have a one-piece or wall-hung toilet
• The shut-off valve is stuck, seized, or leaking
• You’ve attempted a repair and it’s still leaking
• The toilet is an older or non-standard model

Estimated cost: $150–$300, depending on complexity and call-out fees. Regional areas may be higher.

A leaking flush valve isn’t an emergency, but it’s not consequence-free either:

• Water bills – A running toilet can add $50–$100 per quarter to your water costs.
• Floor damage – Attempting tank removal on a corroded toilet risks cracking the porcelain. Water damage to subfloors and ceilings below gets expensive quickly.
• Time – A straightforward repair takes under an hour. Fighting rusted bolts can turn into a full weekend project.

If you’re confident with basic tools and your toilet is in reasonable condition, this is one of the more satisfying DIY plumbing jobs. If anything looks dodgy, especially those tank bolts, get a quote before you commit.