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You empty the system to
A, Check if there is any gas left indicating leaks or neglect
B, so you can remove all the moisture and old oil
C, So you start with an empty system so you know exactly how much gas to add
D, So you can put the system under vacuum to check for leaks
Pressure bears no relation to the weight of the gas and without removing the gas from the car you have no idea of how much is in there before you start
The guy put in 700grammes of coolant in my car thats from empty, he said most cars use 600-700grammes
Too much or too little refrigerant and you could knacker the compressor.
You pays your money and takes your chance
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