Another R-134 Question.

[imported_Big Red]

I have read all of the threads and still I am stupid about the A/C system. Some one will have to make it very simple, remember I was educated in Alabama. How do I evacuate my system for 1 hour? Is there a machine to do this? My system had pressure but when I removed it, I accidently let it out, oooops! I have replaced the dryer and should I replace the expansion valve? I am going to change the oil also. Is there anything else? Please remenber I am cheap and I want to do it myself.

[Tad]

As I understand it, and this is only from watching our auto shop teacher help me with mine, using this pretty good sized machine on a cart (see how little I know about this).
Evacuating the sysytem is different than pulling down vacum. Evacuating just salvages what was left in there (if I'm wrong here, remember I know very little too).
Pulling down vacum seemed, as I watched it to be just like an air pump (built into the machine) that removes all air from the system, and at the same time allows you to monitor for leaks, meaning if it won't go down to so and so number over time, it's pulling in air from someplace.

Now if I am all wrong about that, remember I just watched and have no training in this what so ever.

On to your expansion valve:
I put an under dash unit from an 85 into the 70. The expansion valve is up there under the dash with the unit wrapped in some gooey tape/tar like substance.
If your 79 is the same you only want to pull that baby down once, it was a way big pita to do it twice for me. I replaced it as the unit was down for the second time and never, ever, want to pull it out again.
I also removed the blower motor and squirel cage and cleaned about a 1/2 pound of crud and crap out of there also.

[Retro93]

When you pull a vacuum on the system it removes all of the air, and more importantly all of the water. If the water isn't removed properly, the Freon will react with it and eventually destroy your system. As you lower the pressure on the system the water will boil off and be removed through the vacuum pump. Before you start the final vacuum pull, you can sweep the system with a can of Freon. Pull a partial vacuum, then start the engine and turn on the a/c. Open the exhaust port to the system and let MOST of the Freon escape. Close the valve before you run the system dry. Then pull your final vacuum and recharge.

A new drier and expansion valve are needed for the conversion. Another thing to look at is the foam seals on the air inlet door to the system. After 25-30 years they are probably toast. You can buy self sticking foam by the roll at Home Depot. It's a good idea to clean out the evaporator fins too while you're in there.

The systems in these rigs typically hold 36 WT/OZ. of R12. For R134A, you want to add 85% of this amount. Remember..if you add too much Freon the system will not cool. So be conservative when you are adding.

Oh..and be sure and check the condition of the Shraeder valves on the compressor. If they look like they are worn and tired, replace them. The same goes with any hoses. :D

[imported_Big Red]

Thanks!