Absolutely not, you should use that pump. Pull a vacuum with it, fill with HC refrigerant then repeat (vacuum, refill). You dillute down whatever was left in the system the first time around with the second vac and refill. You can even do a 3rd cycle if you want to as long as the only thing going in is HC. The whole "your pump must pull and and hold 29.999 inch vac" is total BS. HC refrigerants are not susceptible to the same moisture issues as R12 or R134.

I use my car's vacuum pump to pull down to ~18" Hg, then finish w/ a hand vacuum pump. On some cars, I have gotten to 29"Hg after just 5 min of hand-pumping. Last time I did my 1985 300D and could only reach 22"Hg. Might be my current HF pump isn't as good as my old Mighty-Mite (broke). I leave it sitting for hours while I do other chores, to vaporize any water and check for leaks. Some say that isn't a sufficient leak check, but is better than nothing and I won't fill refrigerant if it won't hold a vacuum.

With my car in the sun in July here, it probably reaches 120 F in the evaporator and 160 F in the condensor (can't hold a shiny wrench that was sitting in the sun), so 22 "Hg might be fine. Water doesn't live long in our summers, even at 1 atm. After filling a bit, I blow-off a little refrigerant (Duracool, legal), which helps sweep any remaining air and water vapor out. Even professionals use a nitrogen sweep when they have trouble getting the water out.

Sweep isn't exactly what we do, but it is a good example.

The pro procedure is a triple break. We reclaim, then evacuate, then fill to 150psi nitrogen, then leak check, then we pull a hard 1500 microns, then break with 150psi again, then pull another hard 1500 microns, then break with another 150psi nitrogen, then down to the full 350microns. Much faster and more sure of success than trying for 350 from the start, and it allows for a sure check of leaks.

Hmmm, well I want to do my GF's C230K as well, and I know that that A\C system has probably been empty for at least a couple years... I think I'll just bite the bullet and buy a proper pump, that way I know all the moisture is out.

As a side note, something I've always wondered. Why do people recommended hanging the receiver drier before recharging the system. Shouldn't pulling a vacuum remove all teh moisture that was absorbed by the desiccate inside? Should I replace it?

Vacuum Pump - AC Vacuum Pump w/ R134A & R12 Connectors

I used this cheapo to do my A/C. It was sold around $12+. It works on Bernoulli's principle with a compressor so it is not very effective. I then use a hand pump to finish it off. I always changed the receiver drier if I open up the system. I have converted and fixed the A/C on all my cars without issue and they all cool excellent.

I have since used a very high end vacuum pump in the laboratory, Leybold D8B ( $2000+ new ) which can pull down to 10^-4 mbar but I find the result is very similar. Disclaimer - I did not flush the system as it is too troublesome. My conclusion is that the quality of the vacuum pump is not very important based on my experience, my experience alone. So take your pick.