Connect your vacuum gauge to the line just disconnected from the first "Y" 3-way connector inside the car.

Pump a vacuum of 10 to 15Hg. If this line shows a leak, it means this line is cracked, likely where it passes between the body of the car and the door. If it does not, the line is OK. Make note of the results and continue.

You can reconnect this line to the "Y" connector.