Fluidmaster 400: Ultimate Guide to Adjusting Anti Siphon Valves

To adjust the anti-siphon on a fluidmaster 400 model, turn the adjustment screw clockwise or counterclockwise. To prevent backflow and ensure the proper operation of your toilet or faucet, the anti-siphon valve must be correctly adjusted.

Welcome to a comprehensive guide on adjusting the anti-siphon feature of the Fluidmaster 400 model. The Fluidmaster 400 is a popular and reliable fill valve used in many toilets, known for its efficient water control and prevention of backflow contamination.

The anti-siphon mechanism plays a crucial role in maintaining the integrity of your plumbing system by preventing the reverse flow of water, which could lead to potential contamination of the fresh water supply.

In this guide, we will walk you through the step-by-step process of adjusting the anti-siphon feature on a Fluidmaster 400 model, ensuring that your toilet functions smoothly and your water supply remains safe.

Whether you’re a seasoned DIY enthusiast or a first-time plumber, understanding how to properly adjust the anti-siphon feature will empower you to maintain your toilet’s performance and safeguard your household’s water quality.

Understanding Anti Siphon Valves

Anti-siphon valves are devices that control the flow of water through piping to prevent backflow. The fluidmaster 400 model uses an anti-siphon valve to ensure a steady flow of water to your toilet, preventing contamination and maintaining proper pressure.

It is important to adjust the valve correctly to avoid any damage to the system. The anti-siphon valve of the fluidmaster 400 works by allowing air to enter the inlet when the water flow is interrupted, preventing a vacuum from being created.

To adjust the valve, it is necessary to turn the screw on top of the cap, loosening or tightening it according to the needs of the system. Adjusting the valve guarantees the smooth functioning of the toilets in your home.

The Guide: How to Adjust the Anti-Siphon on a Fluidmaster 400 Model

The Fluidmaster 400 model is a widely used fill valve in toilets, known for its reliability and efficient water control. One of its key features is the built-in anti-siphon mechanism, which prevents the backflow of water and helps maintain the integrity of your plumbing system.

If you’re looking to adjust the anti-siphon on a Fluidmaster 400 model, follow these step-by-step instructions to ensure your toilet functions properly and your water supply remains uncontaminated.

Step 1: Turn Off the Water Supply:

Before you begin any adjustments, it’s crucial to turn off the water supply to the toilet. Locate the shut-off valve, usually located on the wall or floor near the toilet, and turn it clockwise until it’s fully closed. This will prevent water from flowing into the tank while you work.

Step 2: Empty the Tank:

Flush the toilet to empty the tank and reduce the water level. This will make it easier to access and adjust the components inside the tank.

Step 3: Remove the Tank Lid:

Gently lift and remove the tank lid, setting it aside in a safe place. This will give you a clear view of the inside of the tank and the Fluidmaster 400 fill valve.

Step 4: Locate the Anti-Siphon Cap:

Identify the anti-siphon cap on the fill valve. It’s usually a cylindrical component located on top of the fill valve assembly. The cap is designed to prevent the backflow of water and maintain a one-way flow.

Step 5: Adjust the Anti-Siphon Cap:

Using an adjustable wrench, carefully grip the anti-siphon cap. Turn the cap counterclockwise to loosen it. Gently lift the cap off the fill valve to expose the internal components.

Step 6: Check the Anti-Siphon Height:

The Fluidmaster 400 model has an adjustable anti-siphon height, allowing you to control the water level in the tank. The anti-siphon cap will have different height settings indicated by notches or markers. Check the current setting to ensure it’s aligned with the appropriate water level mark.

Also read: Best Price On TOTO Toilets | TOTO Toilets Reviews in 2023

Step 7: Adjust the Height:

To adjust the anti-siphon height, use a flathead screwdriver to rotate the adjustment screw located under the anti-siphon cap. Turning the screw clockwise will lower the water level, while turning it counterclockwise will raise the water level. Refer to your toilet’s manual or Fluidmaster’s guidelines for the recommended water level setting based on your toilet model.

Step 8: Reassemble and Test:

Once you’ve adjusted the anti-siphon height to your desired level, carefully place the anti-siphon cap back onto the fill valve assembly. Secure it by turning it clockwise with your adjustable wrench. Reattach the tank lid.

Step 9: Turn On the Water Supply:

Slowly turn on the water supply by turning the shut-off valve counterclockwise. Allow the tank to fill, and check for any leaks around the fill valve and other connections.

Step 10: Test Flushing:

Flush the toilet to ensure that the water level in the tank and bowl are appropriately balanced. Listen for any unusual sounds and check for proper water flow.

Also Read: Fluidmaster 400 Model Toilet Fill Valve Review: A Comprehensive Guide!

Steps For Adjusting The Fluidmaster 400 Anti Siphon Valve

Adjusting the anti-siphon on a fluidmaster 400 is simple. Firstly, you need to gather the necessary tools such as pliers and a screwdriver. Next, turn off the water supply to the valve. Then, remove the cap and adjust the height according to the instructions.

After that, adjust the water level by twisting the rod slowly. Finally, replace the cap and test the valve. These steps will help you successfully adjust the anti-siphon valve on your fluidmaster 400 model.

Frequently Asked Questions On How Do You Adjust The Anti Siphon On A Fluidmaster 400 Model?

What Is A Fluidmaster 400 Model?

The fluidmaster 400 model is a toilet fill valve that fills the tank quickly and quietly.

Why Is Anti-Siphon Important?

Anti-siphon is important to prevent contaminated water from flowing back into your home’s water supply.

How Do I Know If My Anti-Siphon Needs Adjustment?

Your anti-siphon may need adjustment if your toilet is constantly running, or if you hear hissing noises.

How Do I Adjust The Anti-Siphon On My Fluidmaster 400 Model?

To adjust the anti-siphon on your fluidmaster 400 model, turn the adjustment screw clockwise to decrease and counterclockwise to increase the water level.

Can I Adjust The Anti-Siphon Myself?

Yes, you can adjust the anti-siphon on your fluidmaster 400 model yourself by following the manufacturer’s instructions carefully.


Adjusting the anti-siphon on a fluidmaster 400 model is a simple process, but it is crucial to ensure the proper functioning of the toilet and avoid costly repairs in the future. Whether you are experiencing a phantom flush or just want to adjust the water level in the tank, following the instructions outlined in this guide can go a long way in extending the life of your toilet and optimizing its performance.

Remember to turn off the water supply before attempting any adjustments, and be sure to test the flush and refill after making any changes. With a little bit of patience and attention to detail, you can easily adjust the anti-siphon on your fluidmaster 400 model and enjoy a well-functioning toilet for years to come.

Don’t forget to take good care of your toilet and replace any worn-out parts as needed to ensure the longevity of this essential household fixture.

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