⚠ Safety note: This guide involves working with plumbing. If you're not completely sure about every step, stop and call a licensed professional. Before you start, always switch off the power at the breaker or close the main water/gas valve.
1 Prepare for safe work
Close the corresponding valve, turn off the electrical power supply to the device if any, and wait for the water to cool. Protect the floor with a bucket and cloths, and photograph the initial position of the connections.
⚠ Warning: Tighten plastic nuts by hand and only slightly tighten with a tool; excessive force can crack the cistern.
2 Determine the pattern of the problem
Close the inlet, empty the cistern, and photograph the height and position of the old valve.
3 Check the most common cause
Disconnect the hose while holding the internal part, remove the nut, and clean the seat without scratching the ceramic or plastic.
4 Apply the appropriate solution
Install a compatible valve with a new gasket, adjust the level about two centimeters below the overflow, and check several filling cycles.
? Tip: Photograph the initial state and change only one thing at a time so that you know which check really affected the result.
5 Test the result and set limits
Assemble the connections without overtightening, slowly open the water, and monitor for leaks, pressure, and drainage for at least a few minutes. If the problem returns, close the inlet and call a plumber.