1999 Olds Aurora AC

General Motors is by far the best company at separating owners from their money after the sale, but this should be able to be handled by any independent shop that works on air conditioning systems. If all that's wrong is blockage in the system, that's where a pressure drop will occur and the refrigerant will get very cold. That is supposed to happen in the dash board. Look for frost buildup on a hose under the hood after the system has been operating for a few minutes. The frost will develop right by the restriction.

I'm not sure what is meant by "when the compressor turns on the line pressure goes up but should go down". Both sides of the system are at rest with equal pressures when the system is off. When the compressor runs, it draws down the pressure on the low side and builds it up on the high side. Those two sides are separated by the valve just before the dash board.

If the system is just low on charge it will exhibit many of the same symptoms as blockage in a hose. Checking pressures is not a real accurate way to determine the state of charge. Chryslers use a sight glass to allow you to look for vapor bubbles where there should be just liquid. On GM cars, the only way to know for sure if the system is low is to drain all of the old refrigerant out, then pump in a measured amount. Many mechanics get a feel for the state of charge from experience and the pressure gauge readings, but even they can be mislead once in a while.