Refrigerant does not get "siphoned" in from my observations. If you watch your manifold gauges while charging, the suction side pressure in the system is 30 to 50 psi positive pressure, not a vacuum. The static pressure in the R134a can is around 70 to 80 psi depending on outside temperature. The refrigerant is slowly pushed into the system because it is at a higher pressure, not siphoned in, that could take a long time... like 20 minutes or more. If you invert the can and let the refrigerant trickle in as a liquid a little at a time, you can speed up the charging process.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked