i was wondering if any one on the list might know of a valve that would
function like the cap on the wasserboxer coolant tank.  a valve that would
vent somewhere around 15 psi and also include another valve/orifice to
allow coolant uptake as the system cooled. i've been able to find a
listing for a coolant expansion valve for a porsche 928 but i don't know
exactly what it is or how it works.

thanks in advance for any leads
jonathan