A toilet that runs constantly is annoying enough that people fix it. The expensive one is the toilet that runs silently — a slow leak from tank to bowl that you never hear, wasting anywhere from 30 to 200 gallons a day. On Chicago’s metered water billing, that can quietly add $40–$100 a month.
The usual culprit is a worn flapper: the rubber disc at the bottom of the tank that lifts when you flush. Chlorinated city water hardens rubber over five to seven years until the flapper stops sealing. Water seeps into the bowl, the fill valve tops the tank back up, and the cycle runs all day, every day.
The two-minute dye test
Drop food coloring (or one of the dye tablets we leave behind on service calls) into the tank — not the bowl — and wait twenty minutes without flushing. Color in the bowl means the flapper is leaking. No color, but the toilet hisses or cycles on its own? That points at the fill valve or a tank water level set above the overflow tube.
A universal flapper costs about $15 at any hardware store and replaces in ten minutes with no tools: shut the supply valve at the wall, flush to empty the tank, unhook the old flapper from the flush lever chain and the two ears on the overflow tube, and reverse. Match the chain slack so the flapper seats fully but still lifts clear on a flush.
When it is not the flapper
If a new flapper does not stop the dye test, the seat it closes against may be pitted, or the fill valve is failing — both still inexpensive parts, slightly fussier swaps. And if a toilet needs its third repair in a couple of years, the honest answer is sometimes a new water-saving unit: modern 1.28-gallon models flush better than the 3.5-gallon antiques in many Chicago two-flats and pay for themselves on the water bill.
Either way, this is the cheapest category of plumbing problem there is — as long as it is not ignored. Two hundred gallons a day, every day, is a swimming pool a month through a part that costs less than lunch.