If air is in the system u have to get it out. Now I'm saying this all based on what u call can starting the ac system. But if u don't have a machine, u can charge it and when u burp the air out its with a flat head. Just nub down the valve to purge the air out. Then the pressure would have dropped when u crank it back up after purging the air. Then put freon in to bring the level back up. This is all done with the compressor on the truck. Ur forcing the air and moisture out and replacing it with liquid freon. U don't need a vacuum pump and all that nonsense to get air! It's real easy to do.
I think there's a bit of confusion here about what a vacuum actually does in an automotive A/C system.
Yes, it's entirely possible to recharge an A/C system yourself, and sometimes simply adding refrigerant will get it cooling again. But that doesn't mean evacuating the system is unnecessary.
The issue isn't just the air inside the system. If the system has been open to the atmosphere—or if air has entered during improper servicing—the PAG oil will absorb moisture because it is highly hygroscopic. Depending on the conditions, moisture can also be present elsewhere inside the system. That moisture needs to be removed because it can reduce performance, promote corrosion, and in some cases damage components.
So the question is: how would simply pressing a valve remove the moisture that's already dissolved in the oil?
A proper A/C service follows a proven procedure. First, the refrigerant is recovered. Then a deep vacuum is pulled. This removes non-condensable gases and, more importantly, lowers the boiling point of water so that moisture can evaporate and be extracted from the system. As an added benefit, holding the vacuum also provides a useful preliminary leak check. Finally, the system is recharged with the correct amount of refrigerant and, if necessary, the correct amount of oil.
Of course, an experienced technician can often look at the high- and low-side pressures and recognize that an otherwise healthy system is simply low on refrigerant. In those cases, topping it up may restore normal operation. However, that's still an estimate—it doesn't tell you exactly how much refrigerant is in the system.
In my experience, many A/C problems are indeed caused by low refrigerant. But I've also seen plenty of systems with leaks, faulty pressure switches, failed magnetic clutches, defective control modules, bad relays, sticking expansion valves, or other issues. On an older vehicle with an unknown service history, simply adding refrigerant without first evacuating the system can easily overlook the real problem.
So if a system contains air, it usually means it has been opened at some point or serviced incorrectly. In that situation, venting air through a service valve doesn't remove the moisture that has already been absorbed. Pulling a proper vacuum is still the correct procedure.
I'm not saying that topping up refrigerant can never work—it certainly can in the right circumstances. My point is simply that venting air through a valve is not a substitute for evacuating the system when moisture may be present.
Experience is valuable, but following the correct service procedure is what gives consistently reliable results.
