Anyone having a lack of coolant on warm days

[kmer]

The trapped bubbles do "shrink" as pressure builds in the system, that is, until - as you point out - design temp is exceeded. But thats exactly what happens when air is trapped in the front exchanger - it negates the cooling effect and adds to temperature rise causing higher pressure which in turn causes the cap to open. Now that the coolant level is at its highest and the pressure is dropping (due to open spring)... the air wants to expand - forcing coolant out of the overflow.

 

[LazyBastard]

That must be one hell of a big air bubble to block enough of the surface area inside the heat exchanger to affect cooling.... Also, the temperature light is supposed to come on before the temperature rises enough for the vapour pressure to exceed the cap spring.