I have a Vaillant Turbomax Plus boiler, fitted in 2005. Recently it's locked out several times due to low pressure, which prompted me to check the expansion vessel. When I pressed the Schrader valve I found that water, rather than air, was released. That suggests to me a failure of the diaphragm? I read somewhere that it's often easier to have an external expansion vessel installed than to replace the original - does that make sense to the experts?