I generally don't notice a performance problem with the boiler, but after I run the heat to the radiators and turn the boiler off, after cooling down the the pressure gauge shows zero. I can easily top it up to the "green zone", and all is well. I don't always remember to do this, and it doesn't seem to be a problem. Next time turn the heat on, the gauge is back up to a normal level while heating. In other words, I've never seen the gauge read zero when the boiler has been running.
I don't see any signs of a leak anywhere near the radiators, and it doesn't seem to happen when using hot water -- though this may because the boiler is not running long enough.
Any ideas of what to check?