Al, looking back I see that you have a 23# pressure cap on the engine, what pressure cap do you have on the overflow bottle?   I need to pick up some pressure caps.  JohnD

John;

23# on the filler neck; 15# on the overflow/expansion (OE) bottle.  There is typically close to a quart of air in the OE bottle when cold, which gets compressed as the temp goes up. High power, temps high/going up I typically see maybe 30#; low power, temps dropping it’ll go down to 7 – 8#.  Pressure testing it will go up to 40#.

Al