assemble your system ,If you feel the need to flush ,use refrigerant,just dont get caught venting it.A deep vacume is not a good leak check,it only proves you can hold atmospheric pres. (about 15 psi).Use nitrogen to leak check and blow it out.Run the vacume pump for an hour or so if you have a micron gauge use it go to 50-400 microns.DO NOT USE COMPRESSED AIR IN A REFRIGERATION SYSTEM.
You answered your own question about VENDORS WHO SELL FLUSH STUFF.A shoe salesman wants to sale shoes.before you go thru all the multiple evacuation stuff I would keep in mind that this is a 20 year old truck ,not the space shutle.All that stuff is my opinion ,this is not: the only thing you want in a refrigeration system is refrigerant and refrigerant oil,use nitrogen not comp. air cause it has no contaminates and the vacume pump can get it back out with no problems.There should be some kind of filter dryer to add to the system to help catch anything left in the system that could be removed and replaced if need be. ASE probly has some good guide lines for this on there web site ,if not I do I could e mail it .Good luck.