Thank you Jeff, on these screw head I actually added some JB weld as the counterbore had made a gap under the chest cover and I filed off level.
is it actually possible that air could be creeping up the fixing studs then, I assumed as they were under the plate it would be sealed.
i haven’t tried sealant yet, but that’s only because the valve isn’t fully set yet.
a leaking valve cover isn’t an issue that can’t be fixed from your suggestions, I was trying the air route to put my mind at rest about the valve chest itself wasn’t leaking and worse still the cylinder saddle as both can’t be removed now.
as the sealant can be easy cleaned off I will try a thin layer tomorrow and see if it’s a fix, I did add some washing up liquid around the area and it does seem to be out the top where valve chest cover fits.