My coolant overflow isn’t working.

It’s apparent that the coolant overflow on my Westerbeke 71-C Four has not been working for some time and when things heat up and the coolant expands, it is escaping from around the rim of the cap rather than going out the pipe to the overflow tank.

After replacing the cap didn’t work, I removed the brass elbow you can see in the above photo and found that it was solidly plugged with a bluish white substance that, with a little effort, I was able to clean out.

Alas, it’s still not working.

Here’s a close up of the hole with the elbow removed.

And another from the inside.

As you can see if you look closely, there is a small hole that looks as though it is supposed to connect through to the other side where the brass elbow screws in.
I have been reluctant to use too much force in trying to open things up because in my ignorance, I am not absolutely certain that it’s the right thing to do and I'd really rather not have to buy another exhaust manifold/coolant reservoir ($500) because I really screwed things up.

What I am looking for is someone who knows more than I do to tell me to go ahead and perhaps to recommend a method (forcibly open it with a nail or even a small drill, or perhaps trying to clean things out by injecting a mild acid such as phosphoric acid).

Thanks in advance, Dan