You have a custom system, and so the quantity specification isn't of much use. I would suggest using a set of gauges and trying to match the proper high and low pressure side readings. I'll bet if you get gauges on it you'll see that the low side pressure is running too low due to insufficient freon.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles