Help me understand low side pressure

[sec1rlr]

I have what might seem like an odd question but one that has puzzled me for many years. I know that when charging a system with guages a freon can will empty into the low side. It may take a while, but eventually the can of freon will empty into the low side given the system has some charge and the compressor is running etcetera. Here's where I scratch my bald head. If the low side has 20, 30 or 40 lbs of pressure on it as indicated by the guage, how can the can empty? Logic tells me a new can of freon has way more pressure than that but at some point the pressures should equalize and the remaining freon would not be able to leave the can.
I'm sure there's a simple answer to this, but I'm not smart enough to figure it out. Can someone share with me and possibly others, how a can of freon can empty into a line that has pressure on it? Thanks in advance for sharing your knowledge.

[Tim]

Any time I added refrigerant from a tank or can via the low side, the low side gauge would go up until I shut the gauge or the can tab off.

[JohnHere]

In addition, the pressure in the can is greater than the low-side pressure, so the contents of the can will flow from the higher can pressure into the lower low-side system pressure.

Bear in mind that the pressure in the small 12-ounce can is about 60 PSI, and the nature of refrigerant is such that the pressure will be maintained until the can is just about empty. So, for example, if you're charging a system into a vacuum with the engine running about 1,800 RPM and the A/C system turned on, when you open the valves (on the low side only ), the contents of the can will begin to flow into the system. The compressor will engage after the system pressures overcome the pressure thresholds of the low-side and/or high-side safety switches. In other words, the switch(es) will close, "telling" the ECM/PCM that it's now safe to engage the compressor. As soon as the compressor engages, the low-side pressure will drop below the pressure in the can and refrigerant will continue flowing into the system until the can empties.

Problems result when trying to charge a system in cooler or cold ambient temperatures, which isn't recommended to begin with. In such temperatures, the refrigerant in the can will also be cooler or cold, hence reducing its pressure. In such a scenario, the can might have to be heated a bit (by setting it in a bucket of warm water, for example) to raise its temperature and internal pressure so that the refrigerant will flow into the system.

[sec1rlr]

Thank you both respondents. I didn't realize the can itself would remain at about 60 psi until it was nearly empty. This in itself explains how it can overcome the low side pressure until it is in fact empty. I just assumed it was a linear decline in pressure inside the can.