A cistern that won't stop filling, or one that trickles quietly into the bowl between flushes, is one of the most common calls a plumber gets - and one of the cheapest to fix, provided the right washer goes back in. The catch is that cistern washers aren't a generic size: a headseal or seating washer is shaped to match the valve it seals against, and that shape varies by cistern brand and model. Fit the wrong profile and the cistern will keep running (or keep trickling) even though the washer looks like a reasonable match.
Which washer has failed: inlet or outlet?
There are two washers in play, and they fail differently:
Inlet (float) valve washer - this seals the valve that lets fresh water into the cistern to refill it after a flush. When it wears, the cistern won't stop filling, or it keeps topping itself up on a slow trickle even when no one has flushed. This is the washer inside the inlet valve itself, sometimes called a headseal washer, and on older Caroma cisterns (1995-2005) it's paired with a ballcock washer.
Outlet (flush) valve washer - this is the seating washer that seals the bottom of the flush valve against the cistern outlet. When it perishes or takes a set, water seeps continuously from the cistern into the bowl - you'll usually hear a faint trickle or see movement in the bowl water with no tap or cistern running.
If you're not sure which one it is: dry the cistern and outlet, then watch where the next drip appears. Water pooling in the cistern itself points to the inlet valve; water appearing in the bowl with the cistern otherwise idle points to the outlet seal.
Inlet valve washers by brand
FIX-A-LOO headseal washers are stocked to match the inlet valve body, not a generic thread size:
Trafalgar inlet valves, Marbletrend, Endura, Geberit, and Boston's Fairmont valve each take a different headseal washer. Older Caroma cisterns built between 1995 and 2005 instead use a red ballcock washer - stocked as #2 or #3 depending on the valve, sold individually or as a pack of 2 or 10 for a trade van restock.
Outlet valve washers by brand
The seating washer on the flush valve is matched the same way: Imperial (#1 and #2), Marbletrend, Geberit, Caroma & Dux (also available as a 10-pack), K and K2 valves, Gemini #2, Doulton, Hush Flush, and older Brent-style outlets are all stocked as separate profiles.
Two extra parts cover the jobs a straight washer swap doesn't: on concealed, in-wall cisterns, a bellow washer (sold in pairs) or the Water Wafer outlet valve seal replaces the flexible boot that a wall-hung system uses instead of an exposed flush valve. On close-coupled suites, a foam seal (105 x 12mm or 83 x 18mm) sits between the cistern base and the pan - if that seal has gone, you'll see water at the join rather than in the bowl. A combined Caroma & Geberit repair kit bundles the seating washer and seal for those two brands in one pack.
Why the brand match matters
A generic rubber washer will often physically fit into the valve body, which is exactly the problem: it can look installed correctly and still not seal, because the seating face it presses against is a different depth or profile between manufacturers. That's a callback, not a fix. Matching the washer to the cistern brand - stamped or moulded onto the inlet valve or flush valve body on most cisterns - is what makes a $5-$15 part actually stop the leak on the first visit.
When it's time to replace rather than repair
A washer swap only helps if the cistern body itself is sound. If the cistern is cracked, the brand is obsolete enough that no matching washer is stocked, or a commercial washroom needs something more vandal-resistant than a standard domestic cistern, see our cisterns range for Caroma Uniset II retrofit units and 3monkeez' WELS 3 Star stainless steel commercial cisterns. If the fault is specifically the inlet valve rather than the washer inside it, our guide to bottom entry vs side entry cistern inlet valves covers choosing a full replacement valve, and our mini cistern stop valve guide covers the isolation valve feeding the cistern if that's where the leak actually is.
Frequently Asked Questions
How do I know if it's the inlet or outlet washer that's worn?
Dry the cistern and the bowl, then watch where water reappears first. Water pooling inside the cistern or the cistern refilling without a flush points to the inlet valve washer. Water trickling into the bowl while the cistern stays idle points to the outlet (flush) valve seating washer.
Can I use a generic washer instead of one matched to my cistern brand?
You can fit one, but it's likely to leak again. Inlet and outlet washers are shaped to the seating face of that manufacturer's valve, and a washer that's the wrong profile can look correctly installed without actually sealing - which usually means a second visit to do the job properly.
What's the difference between a headseal washer and a ballcock washer?
Both sit inside the inlet (fill) valve. A headseal washer is the standard part used across most current inlet valve brands (Trafalgar, Marbletrend, Endura, Geberit, Fairmont). A ballcock washer is specific to older Caroma cisterns made between 1995 and 2005, and is sold as #2 or #3 depending on the valve fitted.
My cistern brand isn't listed here - what do I do?
Check the brand name moulded or stamped on the inlet valve or flush valve body (not just the cistern lid) and get in touch with our team with that detail - most current and many older Australian cistern brands have a matching FIX-A-LOO washer even where it isn't listed on the collection page.