The cooling system pressure cap is designed to let vapor escape when the pressure in the system exceeds something like 15psi. That is how air is removed from your cooling system as you run the engine. Just keep adding more coolant to the reservoir as necessary when the engine is cool. On the Promaster the tank is pressurized where on older cars it was not and new fluid was added through a suction hose when the system cooled down. In either system it is going to take a number of warm/cool cycles to get all the air out.Gurgle into the expansion tank?
BTW, one great feature is that the pressure cap regulates maximum pressure on the entire new loop we have added. I remember one poster theorized adding a heat exchanger in the loop so that only non-toxic coolant would flow through the Isotemp. While that would be a theoretical advantage it would also require adding a coolant pump and pressure relief on the secondary loop, a complication and expense I didn't want to deal with. I think the Isotemp heat exchange tubing is very sturdy and each time you winterize your domestic water system and drain the Isotemp tank you are effectively testing that transfer loop for leaks (you would see pink liquid draining from your HWT if there was a leak). The engine coolant system is at about 15psi and your hot water system around 40psi so if there is ever a leak water will quickly be added to the engine coolant system and overflow. With any system like this your need to keep an eye on things.