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.

[gcburdic]

ttt

[RichardJ]

I usually get some rubber hoses that fit tightly on the inlet and outlet of the evap. With both hoses held above the evap, I pour the flush in and let it soak for awhile. After the soak, I lower the outlet hose into something to collect the flush. It isn't all going to come out on its own. You will have to blow it out so the collection container needs to be something that can accept the flush and a big burst of air without spraying that crap all over everything and yourself.
If the flush comes out dirty, you will need a second flush. Then it's a matter of blowing air in from alternating directions until you think it is all out and dry..

[brianthelion02]

^ Worked great for me! 47 degree air feels wonderful!

[ChevLoRay]

to our world! Did it finally get hot enough in Nebraska for y'all to want AC?

[terry b]

What are you using to flush with? I use RX-11 flush. I would like to know what other people are doing. I never use compressed air on a/c system's only compressed nitrogen. A 40 cube tank cost $16.00 to fill . Cheep/ dry/clean and at 3000 psi you will have to use a regulator. Mine goes from 0 to 500 lbs. I also use it to pressure test the system. Take your time and lube all the o-ring's with your refrigerant oil as you put it back together. Then vac it down good after your sure the pressure test hold's.

[RichardJ]

You are right terry, but I don't have a nitrogen tank so I do something else that I am not supposed to do. I purge the system with the refrigerant to push the air out before I close it up to start pulling the vacuum. It all depends on where the service ports are located as to how I do it. I make every effort to expell a minimum amount into the atmosphere.

[terry b]

You got to do what you got to do. How do you do your pressure test with out a stable gas?

[brianthelion02]

I figured if it holds vacuum overnight,it should hold the pressure,guess i have been getting lucky....

[mrein3]

Originally Posted by 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.

Last summer I completed the rehab on my stock AC system. I used all stock parts off of junkers and converted my non-AC cab to AC.

I bought a gallon of the cheapest mineral spirits they had in stock at the local hardware store. For the evaporator core I poured some mineral spirits in one side and blew it out with compressed air from my air compressor with a rag over the other end to catch whatever came out. Then I did it over pouring the mineral spirits in the other end of the evaporator. I did the same for the condenser. This is a MESSY job but after two or three flushes I felt good about the job and moved on.

Don't worry about getting all the mineral spirits out because when you pull a vacuum before you charge you'll boil away any remaining mineral spirits. (Remember high school chemistry. As the pressure decreases, the temperature at which a substance boils decreases. The vacuum not only gets the air out of the system, it boils all the water out as well.)

For the compressor you want to flush it with compressor oil. I bought 1/2 gallon and used about half of that between flushing the compressor and filling the system with the required 11 oz. of oil. The book I read said to measure 6 ounces of oil, pour it in the compressor, spin compressor by hand, and measure the oil that you pour out. You should try to get out as much as you put in. When I was done with the compressor I stood it up to let the remaining oil drain out.

After flushing I would go through the entire system and replace ALL the o-rings. Not just the easy to get to ones.

Once everything is buttoned up, pull a vacuum. Somewhere around -28" Hg you boil all the water in the system. After you get it down to -28, close your system off and go away for 30 minutes. If you have the exact reading on your vacuum gauge as when you shut off the vacuum source, you're good to go. If not you need to find and repair the leak.

DO NOT skip the evacuation step. I picked up an electric vacuum pump from harbor freight for about $100. It got me down to -28" Hg in about 3 minutes. The pumps that run off your air compressor take a lot of air which my shop can't supply. But if you have a big air compressor those evacuation pumps are on the order of $30.

The last step is then to dump in the right amount of refridgerant and you're good to go.

I posted my AC rehab mini-build a while ago. In there is a list of parts and prices that I used to do my job. Some of those parts may help you out like the o-ring kit for $5. Go here:
http://67-72chevytrucks.com/vboard/s...d.php?t=478564
and scroll down to post #8.

I hope this helps.

[El Campo]

Good info on here!