Fixing a Leaky External Cistern Toilet Flush System (by Capitan Farloc)

Introduction

Got a leaky toilet with an external porcelain cistern mounted on the wall or makes a noise like a ship when it fills? You’re in the right place! In this guide, I’ll walk you through the steps to fix the flush system and stop that annoying water loss. With just a few basic tools (adjustable wrench, pliers, screwdrivers, etc.), a bit of determination, and plenty of patience, you’ll get the job done. Of course, time and patience are always hard to come by—but without them, this mission is nearly impossible!

Which Components Can Wear Out?

f you climb up and take a look inside the cistern from above, you’ll notice two distinct mechanical parts (as shown in the picture):

1. The float valve, simply called the "Float," which controls the filling of the cistern and automatically shuts off once it reaches the desired water level.
2. The flush siphon, also known as the "Flush Mechanism" or "Battery." Many people in Italy refer to it as a "CATIS," after the company that manufactures them (www.catis.it).
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How to Identify the Cause of the Leak

Both components mentioned in the previous section can wear out over time and start leaking. In either case, the effect is the same: you hear water running and see a small stream trickling into the toilet bowl.

A simple way to determine which part needs replacing is by checking the water level above the flush siphon, which has the shape shown in the diagram.

• If the water level rises above the bend (e.g., Point A), the float valve is the culprit and needs replacing.
• If the level stays below (e.g., Point B), the issue is with the flush mechanism, which needs servicing.

Another method to pinpoint the cause is to turn off the inlet valve and wait until no more water flows into the toilet bowl. Then, check inside the cistern:

• If the cistern is still full, the float valve is responsible.
• If it has emptied, leaving only a small amount of water at the bottom, the flush siphon is the issue.

The Float Valve

The float valve (which usually looks like the one shown in the picture) can be repaired by replacing its gasket, but given its low cost, people generally prefer to replace it entirely.
In addition to a poor seal resulting in water leaks into the toilet, the float valve should also be replaced when, during the water loading process, it starts to make an annoying sound similar to that of a ship's siren.

Replacing it is fairly simple:

• First, remove the broken valve by unscrewing the nut that connects it to the external inlet pipe.
• Then, unscrew the nut holding it in place inside the cistern and install the new valve by reversing these steps.

There are two key things to watch out for:

1. The small tube attached underneath the valve: When purchasing a replacement, this tube is often placed on the float rod rather than the valve itself. If that’s the case, you’ll need to:

   • Unscrew the float from the rod

   • Remove the tube

   • Attach the tube to the designated nozzle under the valve

   • Screw the float back onto the rod

   If you skip this step and install the valve without the tube in place, water will spray out of the cistern under pressure each time you flush.

2. Once installed, fill the cistern and check the water level.

   • If the level is too low (you can compare it to limescale marks on the edge), bend the float rod upwards.

   • If the level is too high, bend the rod downwards to adjust it properly.

Servicing the Flush Mechanism

Since the flush mechanism is made of cast iron, it’s quite durable and unlikely to break—though not impossible. Before replacing it, it’s best to check the seal at the bottom of the unit. This is the part that lifts when you press the flush button, allowing water to flow out.
The seal is made of soft rubber, which tends to dry out and crack over time, causing water to leak. Given its low cost (just a few euros), it's worth trying a replacement before considering a full repair.
Replacing the seal is straightforward—just unscrew it from the pin where it's attached. Before installing the new one, I recommend lightly sanding the sealing surface with fine sandpaper (grit 1000 or 1200). This helps remove any dirt or limescale buildup that could affect the seal’s effectiveness.

Replacing the Flush Mechanism

Replacing the flush mechanism is a bit trickier and slightly more expensive, but certainly not impossible.
To remove the unit, you’ll need to unscrew the two chrome rings located underneath.

The Chrome Ring Holding the Flush Mechanism

The chrome ring securing the flush mechanism consists of two parts:

• The lower ring tightens a conical rubber seal, which ensures a watertight connection to the drainage pipe beneath the cistern. By unscrewing this lower ring, the mechanism detaches from the pipe.
• Once freed, the larger ring can now rotate. By unscrewing it, you can completely remove the old flush mechanism for replacement.

When disassembling the unit, pay close attention to how the old seals are positioned. This will help ensure the new seals are installed in the correct order and orientation.

Before removing the flush mechanism, loosen the ring securing the flush button tube at the top of the system. Carefully slide out the tube, making sure not to bend or break it in the process.

Once the old flush mechanism has been removed, simply reverse the steps to install the new one.

I hope this guide has been helpful.
Thank you for reading!

Luciano (Capitan Farloc)