Ain't no air in the compressor unless you let all the refrigerant out and opened the system. The refrigerant is supposed to be a vapor in the compressor. In fact, what you did can result in over-charging the system and potentially allow liquid refrigerant to slosh into the compressor and damage it. Compressors can't compress a liquid. They can only compress a gas. With liquid in the compressor, if you're lucky, it will just lock up and make the belt squeal. In the worst case it can break pistons or valves.
The first thing that comes to mind since you found lower pressure after the service is you didn't reinstall the cap on the service port. Those valves will leak and let the refrigerant escape. Their purpose is to hold most of the refrigerant in while you disconnect the hoses and put the caps on. That's why those caps have o-ring seals in them too.
The next problem is gauges in general. They're basically good for telling you whether you have something or nothing. With the older R-12 refrigerant, it was a dandy coincidence that as long as some of it was liquid and some was vapor, the pressure in the system was the same as the outside temperature. That means system not running, radiator cold, and the system has been off for an hour or more to have time to stabilize. If it was 70 degrees F. Outside, you'd find very close to 70 psi in the low and high sides. The problem is though that doesn't tell you how much charge is in the system. If you were to bleed off some of the vapor to cause the pressure to drop to, ... Lets say, 50 psi, that lower pressure would immediately allow more of the liquid to turn to a vapor. Turning to vapor makes it expand a real lot, and that makes the pressure go right back up to, ... Tada! 70 psi. No matter how much you bleed off, as long as some of the refrigerant is still in liquid form, it will expand and the pressure will stay right at 70 psi. Once you get to the point where 100 percent of the refrigerant has vaporized, that's when the pressure will drop just like when removing air from a tire.
I don't know how closely that applies to the newer R-134, but the same thing happens when you bleed some off, and when you add, within reason. Once you approach total liquid in the system, the pressure will go up if you continue to add. Understanding this was an important issue when trying to figure out if the system was low on charge. Doing that was impossible on GM vehicles. The only way to know was to recover whatever was in the system, pump it into a vacuum for a half hour, (that's to boil and vaporize any water droplets at 77 degrees so they could be pumped out), then pump in the exact measured amount called for. You can't do that with small cans because once you attach the last can, if you only need five more ounces, how do you know when five ounces has gone in? The advantage to working on car AC systems is they are designed to be able to tolerate some overcharge because they know the cars will be bouncing up and down hills and floating over pot holes. Home refrigerator compressors can be destroyed by over-charging the system by as little as two ounces.
Chrysler systems are the only ones you could tell when they were fully-charged. They used a sight glass on the receiver / drier. When the bubbles were gone, the system was full. A lot of Fords in the '80s and '90s used sight glasses too, but they were useless. When the systems were full, there were still vapor bubbles in the sight glass. You could unintentionally seriously over-charge the system in an attempt to clear up the bubbles, but they would never go away.
Besides the service fittings, look at all the hose connections and the bottom of the condenser for signs of oil. Oil circulates with the refrigerant and will show up where there's a leak.
To answer your question about bypassing something to get the compressor to run, sometimes we had to do that when using little cans to get the system to START taking a charge. You can pop the cover off the relay and squeeze the contact, but if your car uses a low-pressure cutout switch, it's easier to just unplug that and jump the two terminals in the connector with a stretch-out paper clip. Be aware though, it is there specifically to prevent the compressor from running with no or low charge. If the charge is low enough, the low side could get pumped into a vacuum, and if there's a leak, air can get sucked in along with the humidity in it. The receiver / drier is supposed to be able to capture up to about ten droplets of water. But you don't want to put all your trust in that. Water circulating in the system will freeze at the orifice where the pressure drops and the refrigerant gets real cold. That will stop the flow of refrigerant for as much as an hour until it gets warm enough for that ice to melt. That freezing will keep repeating until the system is properly evacuated to boil all that water out.
Water also mixes with refrigerant to form an acid which leads to leaking condensers and evaporators. As I recall, mixing water and R-12 makes hydrochloric acid.
I also need to stress that professionals wear gloves, safety glasses, and a face shield when working with refrigerant. It can cause frostbite and blindness. You'll find those warnings on the product you're using too.
Saturday, June 6th, 2015 AT 7:55 PM