Thanks Lynn … this is encouraging; my expansion tank does sit at about 25% full and I’ve never seen any coolant expelled into the catch bottle.

In the car, the pressure bottle is not filled full but is kept at about 25% depth. This allows the bottle to act as an accumulator, like airplane accumulators. Like a spring if you like. So, should cap rating be exceeded, only air will be expended, and not coolant.  So, a temperature increase expands the coolant, and air pressure in the bottle will be at coolant pressure, and when the pressure exceeds the cap value only air is expelled.