I assume that it keeps doing it. After you top up pressure.
You have a leak.
Depending on how quick the pressure drops, faster the drop bigger the leak. Check round all your radiators, inlet/outlet, top and bottom, use your hand, WHEN the system is cold. That way your hand will get wet, when you find a leak. If the system is hot, could evaporate the water, before you get to check. Look for signs of leak, brown staining etc, Might not be much, Could just need a little tightening of a fitting, Careful if tightening a fitting, do not allow other parts to move, cause another leak. Do not over tighten a fitting, worse than under tightening it. ( if under tightened, you can tighten it, IF over tightened, could still leak, but would need the fitting/pipe replaced to fix) or if leaking around the top sealing plug or the top bleed vent plug, (the one that you let the air out of), Plug/seal, might need replacing, quite often, just a rubber ring around the metal bit. Dont forget to make sure all the air bleed vents are fully closed.
SD, November 2011
