Per the manual the charge required for "Overhead System C-K" which I believe is GM speak for the rear system, is 5 lbs 4oz.
There are 15.3 fluid ounces in a pound, with means the ORIGINAL R12 requirement is (76.5 + 4 oz) = 80.5 oz.
My supplier (Mike) at Classic Auto Air says that R134a should be 70-80% of the original R12 charge.
Which translates to: 56.35 - 64.4 oz or R13a for this unit, or 6 12oz cans.
Mike said charging it should take a good 45 minutes. He mentioned adding 2lbs, then 1 oz at a time, watching the thermometers in the vents front and rear for "maximum chill".
It's been in the mid 80s all week, and now it's 65 degrees out!
Last run to the parts store to buy some "chilly juice" and IT'S ON!
