It could possibly be down to the physical properties of the two liquids. Perhaps the PAS fluid gives off more volatile gases than the coolant once it reaches it's boiling point, for which the pressure needs to released.

Hmm, that may well be the case. I know someone here in Prague who has sealed his reservoir cap in his ST with electrician's tape so we'll see if he blows up

why does it boil up ?

It boils because the fluid is in a shallow reservoir sat right on top of a hot engine. I was worried too that it boiling would ruin it somehow but the steering seems fine and the fluid hasn't discoloured.

heat wrap it for track day ?

Now there's an idea. That hadn't occured to me. Nice one Steve.