Air Cab AC System Rebuild... Need help!

[ndhunter]

This is my first post so bear with me. I am fresh out of college and saved up enough to buy a 1971 GMC pickup. It is pretty standard with 2wd and a 350. I grew up on a farm and have general automotive after doing my own repair work through college on my Olds. The pickup is an air cab but came with hoses plugged and no compressor. I have purchased a Sanden compressor, the brackets, hoses, drier, and seals/o-rings. I have taken out the tubes, drier and condenser and cleaned and flushed them. I want to flush the evaporator now but not certain how to do it because the bottom tube comes up and if there are metal flakes I don't think they would come out. If I take the driver side of the evaporator/heater core casing off can I get to the bottom of the evaporator? An suggestions with flushing would be great. Also I have scene some people say to flush the POA valve but I don't know if I should.

I want to flush everything and then put it all together and bring it to my local shop to have it vacuumed, evacuated, and recharged.

If it matters I bought an R12 compressor because I have a case of R12 on our farm.