pressure relief valve and expansion tank

[mencheman]

I have a forty year old gas fired hydronic heating system .
Has worked without any major problem
when to fire it up after replacing 45 feet of fin . the boiler filler automatically through the pressure reducing valve .
The system started go hot and all was fine
My son noticed the pressure release valve opened and discharged
I shut down the system and drained water from the expansion tank that has a diaphragm .
it has its own isolation valve and is located just below the pressure relief valve on the boiler feed side.
Any How , how often do these thing go bad , and how can I trouble shoot this