Air pocket in head?
The pressure you are getting is likely an air pocket in the head that was there when you shut down the engine and is trying to get to the radiator or where ever your cap is. You may have a poorly designed system that is putting air in because it is not completely full when running or you never got all the air out when you filled it this spring. You could also be running high temperatures and boiling the straight water mixture if you have a thermostat in it allowing 212 operating temps. To get air out of the block if that is the problem, jack the car up in front to make the cap the high point and turn on your pump if it is electric. If it is belt driven you have to start the car. I hope this helps you solve or find problem.
Bruce