Most boilers run on one atmosphere (around 14-15 pounds), so 5 is low. Is the boiler turned off? Many, not all by any means, have an automatic fill valve that will maintain it at the set pressure. If it is not on and maintaining temp, then if it is leaking from that valve, you probably need to replace the valve. The valve is designed to drain for two conditions: excess pressure and/or excess temperature. If the boiler is off, it certainly isn't excess temperature. So, if you've determined it really is coming from that valve, and your pressure really is 5#, the valve is probably shot.
If the boiler is on to provide domestic hot water (say from an indirect or a tankless), then it sound like you may have a bad expansion tank. WHen it runs, the pressure rises, causing the valve to open. Then, the system cools off in between (some do, not all), the heating water shrinks, relieving the pressure, and now it is low. Go through too many cycles like this without an auto-fill valve, and the pressure will be too low and you'll have problems with the system producing steam when it wasn't designed for it.