losing all refrigerant over night.

[rpctmc154]

1993 chevy g20 5.7. with rear a/c . pulled a vacuum and got 30in. pulled vacuum for about an hour. let sit for 1 hour with no lost in vacuum. filled with the correct about of refrigerant. a/ c ran nice and cold after sitting over night I have lost almost all of my refrigerant. any ideas of where I may be leaking from that wouldn't show on my vacuum results. there is a small hissing sound coming from the passenger side condenser.which I thought was just the pressure equalization.

[Cusser]

there is a small hissing sound coming from the passenger side condenser.

Do you mean "evaporator", like under the dash?

[rpctmc154]

no the condenser in front or the radiator

[PromiseRing]

It’s very easy for a system to hold vacuum indefinitely, but immediately leak when it sees positive pressure.

You can fill the system with nitrogen and listen for leaks. You could also charge with some UV dye.

[GM Tech]

My money is on the rear a/c lines leaking at a grommet most likely right at the rear wheel. Look for oily greasy leak,,,