Power brake conversion issue

That is very definitely a non functioning compensating port. To prove it release the fluid pressure by cracking a line. Very carefully measure the fluid level in the reservoir. Clean the rubber lid seal, the side that faces the fluid.Put the lid back on.
Now, go pump the pedal until it gets hard and high and no longer accepts pumping action.
Next go remove the lid. Look at the the seal you wiped dry. There should be fluid all over it from the returning fluid spray. Is it dripping? Next measure the fluid level. It should be exactly as you marked it. Is it? Put the lid back on and secure it.
Two no's means the fluid is trapped out there.
Let's get it to come back.

Oh, I see SIX225 already gots you covered!