Agreed,

The coolant pressure gauge readily shows the overnight residual vacuum (~ 1
to 2 inches), air in the system (rate at which pressure rises and max
attained), head pressure vs. rpm, thermostat opening point and the point at
which the radiator cap opens sending excess hot coolant to the overflow tank.
The maximum running pressure also correlates with the coolant temperature.
For example, turning on the AC (in my implementation) also triggers the low
speed fan. This immediately reduces the coolant temperature and the response
is seen with a 2 psi delta. Air in the '82 diesel heater circuit is also
readily found. Equilibrate the system at max pressure, then open heater
valve. No air, no pressure change. Air, pressure drop. Very useful device.

BTW, I recently switched my coolant pressure gauge from the cheap JCW Boost
gauge (~ $15) to the VDO Turbo Boost Gauge. I put in the 0 to 30 inch
vacuum/0 to 15 PSI unit for more precision and got a bonus. The VDO Boost
gauge has a sintered metallic frit at the gauge entrance to eliminate any
possibility of water or coolant from getting into the sensing tube.

Frank Grunthaner