Here is a good site to check out http://www.ackits.com/ in the FAQ section it list the procedure. It does say to pull vacuum (it even say to vacuum it multiple times but can see that a a/c shop would do all this). The site even direct you to where you buy a vacuums pump (don’t know if the pump vendor is any good).

Good luck

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Silver Honda Accord, 2006
Silver G500, 2003
Silver SLK-320, 2002
Black ML-320, 2000
Bule Porsche 993 Targa , 1997
Silver Merkur XR4Ti, 1987