It could also be the expansion vessel, its job normally is to keep the pressure the same no matter the temperature of the water. Basically, it is a vessel with a membrane down the middle, water on one side, air on the other, if the membrane breaks, then it cant equalize the pressure. Do you have an overflow pipe (not the correct name) leading to the outside of the house?, if so fill the water pressure up, then knock on the system and see is the water spurting out through the overflow pipe. If so then it could well be the expansion vessel. If you fill the system up and it is pouring out to the outside of the house(without the system being on) then the probability is that it is the overflow valve that is faulty.
The pilot light cannot be lit while the pressure is so low, this is a safety issue as if it was to boil up and there was no water in the system, then it would burn out the whole boiler in no time.
The pressure relief valve is generally quite easy to find as it has what looks like a key ring attached to it. You pull the ring and it will expel the excess water to the outside of the house. An idea might be to pull it a few times in case there is something caught in the valve itself (fill the system, pull ring, and repeat).