Air conditioner

I never heard of a trapped charge. Discharging the system takes a while to boil off so it make look like it's trapped. Pulling the hose will lose refrigerant and oil, and let air into the system. While the system is empty, add no more than two 14 ounce cans of 134a while checking pressure with a gauge. Make sure fan and compressor are coming on. Or take it to a shop for evacuation and recharge.

I fully discharged it with a robinair ac machine. The gauges read zero and yet when I pull the very first hose off at the condenser it went off like it was fully charged

Like I suggested, when a system is discharged, it takes a while for the last bit of vapor to escape. Refrigerant makes lots of cold while boiling off. When things later warm up, more vapor escapes unless you trap it. Trapped, it again builds pressure. Also, some refrigerant is dissolved in the lubricating oil. It takes time for that to boil out too. Now, here's the short answer: don't worry about it. It's just coming out of where it should be anyway.

I had never seen one fill the garage with a little trapped vapor before. Enough to create a fog that you could barely see through!