A couple of things.  Tracy and I both discovered that once ALL the air is out of the coolant system, the coolant pressure may jump to 21 psi almost immediately upon start up - long before the heat would cause it to build to that point.  At first, I thought "coolant Leak", however, I could never find any evidence of coolant in the chambers nor loss of coolant in the system.  Talking to Tracy Crook, he reported the same thing.  We came to the conclusion that without a small cushion of air that the pressure sensor was seeing hydraulic pressure caused by the pump. 

 

Maybe not.  How/why would the pump pressurize the system?  There will be some pressure differential across the pump, and downstream from the pump you will see the drop around the loop; but not 21 psi.

 

Maybe depends on what you mean by “almost immediately”.  With no air in the system a small amount of expansion will cause a pressure increase.  I recall seeing over 20 psi when the temp was only up to 125F.

 

Al