What does it take to go from R134a back to R12?

You would need to flush the system, change the drier, and add the proper oil for R12. The compressor should probably be drained somehow as well....Rich

It is an all day job for me. Disassemble everything, blow everything out with the proper flush materials, replace the dryer, add the proper amount of mineral oil, make sure mineral oil is in compressor, replace and lube up o-rings and button it up. Vacuum, typically overnight, then charge with the proper amount of refrigerant and you should be good to go. I typically will install a new compressor too.

Plug for Rollguy here. Over past couple years I have gone through one 300D with a new r4 compress and dryer and putting it back to r12. On another 300CD I went with Rollguy's Sanden conversion kit in addition to a new pflow condener and dryer and running r134. Hands down the unit running the Sanden a/c works better than the one running the original r12.

Make sure you address hose issues while the system is apart.

I have the R4 in my 1984 & Sanden in my 1985. Both cool about the same, which is marginal on our >100 F days. I don't know why an R4 wouldn't work as well. Both have many mini-pistons w/ flapper valves. I think people ding the R4 more for durability, which is probably from poor lubrication. Read up on Hella's PAO 68 oil (Duracool or ebay), which stays in the compressor better. I suspect a parallel-flow condenser would help the most. You can tell how critical the condenser is if your temp switch becomes erratic (like mine) and doesn't run the electric fan when idling.

Most R4 compressors fail because of incorrect oil and amount used. Then, there's the crap shoot rebuilt R4s that may work 4 seasons.

The bottle neck in cooling capacity is the condenser. In 1985 the 300SD got an up graded condenser which makes ice. My 85 SD with it's R$ and R134 is a meat locker.

btw, there is an aftermarket P-flow for the W126 that is a direct bolt in replacement. I'm running one in the '84 SD with a sanden pumping R152a, aka dust off, and it cools fine in our 100f Texas summers. I can't even feel the compressor kick on.

I wish the '85 w126 condenser would fit the w123

Tinted windows and get some of that laminate floor underlayment from home depot, the stuff that is like bubble wrap and put under the carpet. There's a lot of heat entering the cabin off the pavement.

This is why I suggest my relay mod, it runs the fan whenever the compressor clutch is engaged.

looks like a great mod Rollguy

Dye may or may not hurt or clog up stuff... but is totally useless with regards to a leak in the Evaporator... thus only a Sniffer is the gold standard.
It is legal to test for leaks using a sniffer with nitrogen and 4 oz of R22 and release into the atmosphere... according to the EPA... they figure that is better than people charging up leaking systems only to lose more than that in the long.. .or shorter run.
Your system needs to be checked pressurized.. that is the way the seals are when working... vacuuming is for the moisture removal.
Nitrogen ... in addition to the pressure checking procedure is used to remove moisture also... triple evacuation on home systems is pretty standard... the dry nitrogen takes up some moisture.. you take it out... repeat.....
check out the AC reference threads in my signature...
Duracool is a BLEND.. thus later if you need to top off the system... all must be removed and filled with new....due to blends leaking their parts at different rates...

Of?

After I pressure test I usually pull a vacuum and let it stand with gauges attached so I can see if there are any changes. I have had systems that are ok on pressure but leak on vacuum.

Read it straight from their OWN page..
DURACOOL - Official Web Site

Exactly.... we have had several report that... thanks for sharing your first hand experience.... I have not had it happen to me... I am going by other's reports like yours...

OK, I have found a guy that will pressure test with nitrogen for me!
Then vac the system
VSTECH said to use 150lbs of pressure with the nitrogen, anything else needed with this?
Dye or no dye?
I am refilling with r12