There are can tappers that have a threaded outlet, and you attached that yellow hose to the can tapper outlet. Once the hose is attached at the can end, leave it loose at the gauge manifold end, and tap the can slowly, allowing some refrigerant to escape from the other end of the yellow hose to purge out the air. Tighten up the yellow hose at the gauge manifold, fully open the can tap, the open the appropriate gauge valve on the manifold to connect the yellow side to the blue side so you charge from the can, via the yellow hose to the manifold through the blue hose.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC