I think John has the answer.
The system sucks in air through the overflow as it cools down. If you will drill a 1/16" hole in the flange around the thermostat. It will act as an air hole and the air can escape from the system without pushing the out the water.

Joe ;\)

My Truck: https://www.inliners.org/ubbthreads...Number=100611&#lg=100611&slide=0

Inliner Member #393