When you started work on the system, was there any pressure in it or had all the gas been lost?

No pressure usually says that you have a good sized leak.

I'm assuming you bypassed the low pressure switch to get the compressor to turn on? As far as vacuum and how long to leave a system pumped down, you need to get really close to a complete vacuum to get all the water out. If its warm out, the water will boil out with less vacuum. At 60?F you need to hit 29.4". It's been a while since i have looked at the charts, but I remember pumping down systems that had been open for two or more hours. On a cool day, I would run the pump for four of five hours.

Also, the AC vacuum pump should be some type of two stage rotary vane pump that's rated for this type of application. Not sure how well the ones that use compressed air work. Most pro sites say they are good for air removal but not enough to remove any moisture. On any system that's totally lost its charge it's possible the you also lost oil.

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Also, if the system lost all its charge or was open its good practice to replace the reciever dryer so that your starting with fresh desiccant.

Kevin