How to replace orifice tube

First the nuts are metric. but you must first evacuate the system to get it empty before replacing orifice 13/16 like you say may be 18 mm and other is probably a 15 or 16mm crowsfoot will work. On one see link.
https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

Thanks a lot!
Yes. I know that many other screws in GM parts were shown with metric. The back up wrench should be the answer for the tool. The Small nut is 19mm and big nut is 23mm at expansion valve connection? I already discharged my system, and doing rebuilding now. It is still not clear about the size.

Thanks for your prompt reply and help!
I was wrong there was no 11/16 nuts. I measured several time in different ways. The small one is 19mm, and the other big nut is around 15/16 inch area and 23 mm in metric.

There is no way exist your mentioned 15, 16, and 18mm nuts on the expansion tube joint nuts on my car. Pointac Montana 2003, and transferred from Canada by dealer. That may be caused differences. Also, the area structure on the expansion tube connection is more complicated than shown on the Hans repair manual. There is T connection.

Thanks! If any techs can help with this topic?

Yes the T connection is there because you have rear air. Proceed as you normally would.

Please let us know what happens.

Cheers, Ken

Hi Ken, Nice to hear from you again. I just could not recognize the two nuts size at expansion tube. I want to buy but do not know the sizes exactly. 15/16 or23-24mm for big, 19mm or 20mm for small one. If Crowfoot Flare Nut Wrench is suitable to release them?
I will appreciate much your help to resolve the problem. I search a lot of sites but could not find it.

My question is if Crowfoot Flare Nut Wrench set can be used effectively release the nuts on the expansion valve to replace orifice tube on Pontiac Montana? Is it right sizes for small nut is 3/4 inch or 19mm in metric and for big nut is 15/16 inch or 23mm in metric. It is really looks like that GM car repair is very difficult. Few people really conducted this job. If anyone on this site could answer the question? Thanks!

I am sorry that I was wrong with orifice tube on Montana 2013. There was no orifice tube when I release this joint with 15/16" and 20" fraction flare nut wrench. There is expansion valve instead of orifice tube in 2013 cars.

O I thought it was a 2003? Yep in 2013 expansion blocks are used. Is there more info needed or?

Yes. How to replace expansion blocks, and where it was located. It seems difficult job. May you send me instruction and manual information please? I have Heinse' repair manual and service manual but I still don't know exactly.
Thanks a lot for your helps!
Steven