The flush valve tells a different story. If water is draining slowly from the cistern into the pan even when nobody has touched the button or handle, the flush valve seal or siphon washer has likely perished and is no longer sealing the outlet at the base of the tank. This shows up as a cistern that refills every few minutes with no obvious trigger, sometimes described by customers as topping itself up rather than running continuously. On older gravity-fed systems built around a siphon rather than a valve, a split diaphragm inside the siphon body produces the same symptom.

The lever type matters too. Close-coupled cisterns almost always use a side lever or a push button through the lid, while older low-level suites more often run a side lever connected by an external link arm. Ordering a replacement lever without checking which type is fitted is a common cause of parts being sent back.

The third check is worth doing before assuming either fault: isolate the water supply to the cistern, remove the fill valve, and inspect the seat and washer directly for scoring, grit or mineral buildup, particularly in hard water areas where limescale narrows the valve opening over months. A valve that looks visibly worn or scaled at the seat is the straightforward fix. A valve that looks clean and undamaged but the noise persists points more strongly toward mains pressure as the underlying cause.

Once inside, the cistern behind a concealed frame is usually a compact, purpose-built tank rather than a standard visible cistern, and the fill valve and flush valve fitted to it are sized to fit that specific frame's depth and width. This is where sourcing spares gets harder than for a standard cistern: frame manufacturers often specify particular valve models to fit their tank dimensions, and a valve that works fine in a standard low-level cistern may simply not fit inside a concealed frame's tighter cavity.

A cistern that never quite stops filling is one of the most common callouts a plumber gets,  [[https://www.yell.com/biz/plumb2u-epsom-10205724/|Plumb2U]] and the fix hinges on correctly identifying which of two components has failed: the fill valve or the flush valve. Get this wrong and you swap a part that was never the problem, then head back out a second time.

The practical check before assuming universal will do the job is to look at how the existing part connects, not just what it looks like. A universal valve that adjusts in length is not the same as one that adjusts in outlet diameter or seal profile, and forcing a universal outlet against a proprietary seal often produces a fit that looks correct at first glance but weeps within days.