I would drop the level until the tops of the flues JUST are uncovered and see how much room you have. Some radiators have more than others. I'd fill it up maybe 1/2" to 1" above the flues and call it full. I bet it's either too full, or your cap is bad, or both. I'd throw that cap in the garbage anyway. It's import junk. You should be able to run a 13 pound cap with no issues. That one is almost 16. The less pressure, the less stress on everything.