Fluctuations on the pressure gauge usually mean that the expansion vessel (the red tank) has little or no air pressure inside. This tank is in 2 sections - air on one side (around 10 psi) and water on the other....if there is a leak, it really should be replaced - cost around £80 for the part. The air side has a normal tyre valve fitted for air replenishment - if you depress the valve and water comes out, the diaphragm that separates the 2 halves of the tank has perforated.
John, March 2008
