Some tanks fail when a leak develops in the diaphragm. This usually causes the tank to fill with water and become “water logged.” You can check for this by pressing in the stem of the valve that pressurizes the tank. If a stream of liquid comes out the tank is time to replace. Tanks can also develop leaks in their thin steel shell. The expansion tank is designed to handle the thermal expansion of water as it heats up in the water heater, preventing excessive water pressure. If water pressure gets too high, it can damage valves in plumbing fixtures, joints in supply pipes, and the water heater itself. Thermal Expansion tanks fail a lot of times due to high pressure. This video shows you how @TappPlumbing checks your water pressure. Definitely start here first:
CHECK YOUR WATER PRESSURE
   • HOW TO CHECK & ADJUST WATER PRESSURE ...  

HOW TO REPLACE EXPANSION TANK
   • Easily Replace a Thermal Expansion Ta...