After having a charge put on my R12 system, 1989 F-150, the compressor spits freon. It starts spittin after about 2-3 minutes of switching on the AC or defroster. It is coming from a fitting on the backside of the compressor.
The air is still not cold from the vents. The pressure reading was 40 psi with the truck running and AC on max.
The compressor also cycles off and on.
Is that a relief valve that is spittin?

 

You need to know what the pressure is on the high side. Sounds like a compressor problem. Also, 40 psi is too high for R-12. You should be around 34 psi. But even at 40, you should feel some cold air. Sure sounds like a bad compressor or plugged oriface.