The toilet runs weak, runs forever, or won't refill: the cistern is four parts

Every toilet complaint in the house lives inside the tank, and the tank holds four working parts, which makes every complaint a four-question interview. Plumbers call a cistern "easy money"; the reason is that nine times in ten the part that failed costs a wallet-swap and the tenth is the job they genuinely need. The interview below is how to know which ten you are in.

Four parts, four symptoms

• The flush is weak or dead: the flapper (or the tower seal). Lift the lid; flush while watching. The seal should lift fully and stay up the whole flush. If it drops early, the chain is too long (adjust to a slack thumb's width - the whole fix, free) or the seal has gone soft and black from the chlorine tablets everyone drops in here (replace it; it is the most stocked toilet part in any hardware shop).
• The tank barely fills: the water level, set by the float. The mark on the back of the overflow tube is the target - the level should sit about an inch below it. A float set low gives a weak flush even when every part works, and a float set high sends all the excess down the tube while the tank "won't fill." One bend or one screw, depending on the era of the mechanism.
• It runs forever after flushing: the fill valve not shutting. Listening at the lid: a hissing tank that never quite stops is the valve's seal dying - and while you are there, the refill tube aimed into the overflow (it should trickle a stream down the tube; if it's detached, the bowl never refills to its smell-blocking level - which is why "my toilet smells after the flush" is a twenty-second fix you can find here).
• Bubbling, glugging, slow drain after a good flush: the rim jets and the trap. The small holes under the bowl's rim are what create the swirl; mineral crust closes them into a dribble - an old toothbrush and vinegar down the ports, held wet with paper towels for an hour, is the treatment. A toilet that flushes clean-but-then-refills-noise-like-a-drinking-animal is a partly blocked vent stack: that's the roof, and that's the plumber.

The two where you stop and call

Water at the base of the toilet after flushing (the wax ring or the flange is failing under the floor, and the subfloor is learning about it) and sewage smell that no amount of scrubbing fixes with the tank's refill and the trap's water level both verified good - that smell is the drain system telling you it needs a professional camera, not a bucket of bleach. Everything above the two is a lid-up afternoon.

The habit that prevents half of them

No chlorine blocks in tanks with rubber seals - they eat the flapper from the soft side, and the shop selling them will not mention it; flush only what the trap was designed for, because the "flushable" wipe earns its name only in marketing; and when the house will be empty for weeks, close the isolation valve behind the toilet and flush the tank nearly dry - a seal that dries under a full tank cracks, and that is the whole story of toilets found dead after every festive-season trip.