When you take the rubber cap off you will not learn much, as you cannot diagnose a leak in your heat exchanger from there. You may find a lot of crud in there from the remnants of your pencil zinc (you are changing those, right?) and maybe some other crud if you are not using some kind of sea strainer, but none of this is likely to make your expansion tank overflow. Be Very careful to align the rib
in the center of the rubber cap with the division in the heat exchanger when you replace the cap. The whole apparatus will not work
correctly if you do not get this on exactly right.
I would not add any more antifreeze
for a period of time and see what happens. My old W50 on my Tartan 37 ran for 3000 hours with the standing water
level in the expansion tank about an inch--maybe more lower than the cap of expansion tank. You might have no problem at all other than the temp being a bit warmer. Start with the cheap
If the expansion tank is filling regularly no matter what the ambient temp, I would say the next step is to remove the heatexhanger and take it to a good radiator shop and have it cleaned and tested. It is no fun removing this, but it is possible. While you are at it check you exhaust elbow
. These are two routine maintenance
items that, because they are a pain in the butt to do, rarely get done often enough.