this is true, however, if the system is equalized and you are reading 75psi at 80°F, then you are certainly low.
at equalized 80°F you should be around 85 to 87 psi depending on the mix of chemicals in the Freeze 12. this is only true, if the car is not hot inside, and it's not been running for long enough to totally soak the temp throughout the system.
so, while it is a pretty useless #, the resting pressure, can tell you at least if you have some liquid in the system. in your case, I doubt it does.
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!