Blowing air through the receiver/dyer was a bad thing. It's now loaded with moisture. While you have it off to replace it is a good time to replace the two sensors mounted to it. You want to move quickly when replacing the receiver/dryer. It becomes contaminated by moisture in the air very quickly. Once you have it back together you also need to connect a vacuum pump and pull a vacuum for about an hour to remove any remaining moisture. If you don't the system will never cool.

When you recharge it with R134a you need about 75% of the charge it had when it had R12 in it. The Original charge was 1300 grams, so 975 grams of R134a should do nicely. That's about 3 ounces less than 3, 12 ounce cans.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire