How To Diagnose a Leaky Toilet

In my previous posts on how to repair a leaky toilet I'd already worked out where the leak was and went straight ahead and replaced the flush mechanism with a water saving dual flush mechanism, although I did mention how you could fix the leak.

I thought I'd better put that right by explaining how to find and fix the most common leaks.

When a toilet starts to leak it generally leaks into the pan and down the drain.

There are 2 types of leak:

1. The toilet is overfilling , causing the water to run out through the overflow tube.(Which on most toilets runs into the toilet bowl ,on older toilets the tube may go straight outside).
2. The flush mechanism isn't water tight causing a leak, also into the bowl.
   So the first thing to do is decide which type of leak we have.

The easiest way to do this is to simply turn off the water supply.

If the toilet is overfilling and water running thru the overflow pipe then once the water level in the cistern has dropped to below the level of the overflow pipe it will stop overflowing. The toilet then stops leaking and the water in the cistern stays at this level. This is leak type 1 and is sometimes described as a running toilet. This indicates a problem with the fill valve or the float.
If , on the other hand the toilet keeps leaking until the cistern is empty then the problem is with the flush mechanism.

Now we know where the problem is – we just need to find it.

Solving Type 1 LeaksIf the toilet is overflowing it could simply be because the fill level is set too high.

In which case we simply need to lower the water level to below the height of the overflow tube. This can be done in one of 2 ways depending on the mechanism fitted. If we have an older ball and rod style float then the level can be lowered by simply bending the rod down slightly.

If we have a more modern style float , as shown in the photo, then the level is lowered using the adjustment screw.


In either case if adjusting the float height doesn't solve the leak then the inlet valve is probably faulty and needs to be repaired or replaced.

Solving Type 2 LeaksIf the flush mechanism is leaking then there are three possibilities:

• The flush valve is obstructed or not shutting fully. (This is especially probable if its a very old toilet where the valve is a simple flap) In this case once the obstruction is removed and the valve can close properly then the toilet will stop leaking.
• The flush valve closes properly but leaks. In this case check the state of the gasket , if its perished or otherwise damaged (as shown in the photo) then it needs to be replaced. Once the gasket is replaced it will stop leaking.
• If the flush valve seals correctly but the mechanism leaks then check the “O”-rings or gaskets which seal where the mechanism attaches to the cistern. If these are perished or damaged they may cause a leak and need to be replaced.

If the toilet is running or leaking into the bowl then one of the above methods should fix it.

If the leak is outwards (onto the bathroom floor) then it could be due to A cracked cistern , The seal between the cistern and the pan, the seal between the flush mechanism and the cistern, or the seal between the toilet and the drain. If its a faulty seal then it will normally leak when the toilet is flushed . If its a cracked cistern then it will generally be a continuous leak. More information about diagnosing external leaks can be found here.