Hi. The most common fault in any water heater is the diaphragm rubber. you must disassemble the body of water or diaphragm holder. I leave part of your photo with the indications of where to start disarming. Once you laugh at the body of water and remove the three screws from its parts you will find the rubber that acts as a diaphragm. You can buy it at any hardware store. Any questions you have more to ask. Regards