If it is a really old system, it may not use a bladder type expansion tank that is more common on newer systems. Older ones may not have an auto-fill valve, so controlling the pressure is dependent on opening the main water feed valve while watching the pressure gauge.
If it has a old water tank for expansion, if there's a leak, it could fill entirely with water, and then wouldn't function as an expansion tank anymore. To restore it, you'd need to shut the water off (and the boiler!), drain the tank, then close the valves and turn on the fill valve until you get the proper pressure (probably somewhere in the 12-18# range).
The pressure relief valve could be shot - the piston on them can get corroded or covered with deposits, and once they open, they won't close again...you'd need to replace it.
Boilers with compromised safety devices can be very dangerous. It may be time to call in a pro.