When you pull down an AC system for recharge....

My vacuum pump pulls the pressure down to about -30 on the low-side gauge in just a few minutes.

But I've always heard you should let the pump run for quite some time.

If the pump has pulled the system to as low a vacuum as it can, what is the point of continuing to let it run? Or is there one?

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2012 E350
2006 Callaway SC560