when you attach the gauges, with the compressor off, the pressure should read around 100 psi if you do not have a leak.
more likely, it's going to either read 0 or really low. if it's reading low, there's a leak, and you have to pump it up with a dry gas to find the leak. I use nitrogen, but you can use co2, argon, helium etc. just NOT COMPRESSED AIR! if it's at 0 you need to add in a trace gas of 4oz R22 to use a electronic sniffer. you can also use soap bubbles and check all the fittings and connections with bubbles... not as reliable but it's helpful for spotting leaks in the o-rings.

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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 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!