Just so you know, I have done a few R-12 to R-134 conversions in 1970 chevelles with factory A/C. I have also done my 1970 SS 454 Sta. Wgn.
Some items that must be changed,,,, If you want the A/C system to function as it should.l
FIRST: The P.O.A. valve MUST be adjusted for R-134.
SECOND: R-134 builds more heat than R-12, I replaced my org. 1970 condenser with a FACTORY FIT -1970 / 72 Parallel Flow condenser. ( This condenser uses ALL the factory nuts,, bolts,, and brackets., I have one on mine) WHY? The G.M. condenser with the Appx. 3/8" dia. tubes WILL NOT cool the R-134.
Third: Using the old style A-6 Compressor is OK,,, BUT you will recharge it just about every spring. WHY? The R-134 molecules are much smaller than the R-12s, So it will leak out of the front seal and the Org. R-12 hoses.
Fourth: You need a dryer that is R-134 compatible.
Fifth: I installed R-12 / R-134 compatible hoses.
Sixth: The org. 1970 C-60 system takes a R-12 charge of 60 Ozs. When you convert a C-60 system to R-134- The Charge of R-134 should be 48 Ozs.
Last year I replaced my A-6 compressor with a new style Alum. compressor that uses " ALL THE ORG 1970 NUTS BOLTS & BRACKETS & HOSES ". It is the same size as the A-6 and designed to operate on R-134
Getting the 1970 style O.E.M. A/C unit to operate as it did in 1970 will cost some money. To convert a 1970 C-60 system to R-134, Be prepared to spend at least $1000.00.
After many years of trying to get cold air CHEEEEEP in my Red 70. I did it right. Having your A/C system to function well,,,, IT IS NOT cheap.
If any one trys to tell you a different story on how to get a 1970 Chevelle A/C system to function well, Also they may say, You do not need to do all of the above.
Myself and other T-Chevelle guys have converted some early G.M. A/C units. It cost a lot of money to be KOOL
Using the above ideas and skipping nothing, I get 38 to 43 degrees out of my center vent and I HAVE NOT touched the A/C unit in over two years.
Again, " Being KOOL Aint Cheap ".
Over the years I screwed around trying to get cold air cheap. So I spent some cash and its over and done with. If you are converting R-134 do it CORRECT or dont even start.!!!!!!!!!!!!!!!
Trivia, To account for a bad seal in heater doors or a bad 1970 Vacuum operated water valve, I went to Lowes and I have a water " Shut Off Valve " mounted near the firewall so you KANT see it, I painted it black.