I made a cheap tool to check out my coolant system.  I took a 5/16" brass
nut and drilled out the center to about 8 mm.  I then bored an 1/8" hole
sideways into one of the flats.  I soldered some 1/8" copper tubing to the
nut and attached a vacuum/pressure gauge to the copper tubing using a
compression fitting.  The nut gets pinched under the radiator bleed bolt
using an extra copper gasket.  With the grill off, I placed the gauge under
the passenger side wiper and started the engine.  Pressure goes up to 3 or
4 psig pretty quickly, and after 5 minutes or so, climbs to 12 to 13 psig.
Revving the engine causes a slight increase, but at steady rpms, it again
levels out around 12 psig.  This is at or near the cap pressure setting.
Sustained high-speed driving would likely push some coolant over into the
overflow tank.  It takes many hours for it to return to 0 psig.  This was
done on my '85, in which I just installed a new engine, water pump,
thermostat (stock temp), all new coolant hoses,  coolant, etc.  Radiator is
full right to the top.  Should be a pretty good baseline test.   The
temperature gauge sits right on top of the light almost all the time,
occasionally climbing one needle width above.  At this point the fan kicks
in and brings it right back.
I think a great spot for a permanent pressure/temperature measurement would
be at the front of the driver's side head.   There is a blocked off hole
that is a just a flat, oval plate with a gasket.  This flat plate could be
tapped for a temp sender and a pressure sender.   1.9l only?   Anyone done
that?

                                                      Jay