Your compressor needs oil to function. The oil used is carried through the system by the refrigerant and chances are you lost some while you were servicing things. If you are doing the R134a conversion, you should remove the compressor and drain all of the existing mineral oil, as it is not compatible with R134a. You should also replace the receiver/dryer. Measure the amount of oil contained in each by pouring it into a suitable container. I was replacing a damaged receiver/dryer so I just measured what was in it and purchased enough to replace it. If you have the compressor or receiver dryer off, you can simply pour the oil into them before installation. Small cans of the proper oil can be installed using the newly created vacuum to pull the oil into the system.

The car should not be running at this point. Using the can tapping valve, connect the service line, opened the valve on the can and cracked the service line connection at the manifold until oil started to leak out to purge any air out of the service line. Holding the oil can upside down (valve down), open the low side valve on the manifold gauge. The vacuum will pull the oil into the system. When you have determined the can is empty, close the low side valve and the valve on the can. You should now be ready to charge the system.