The Fat End with the Check Valve is closest to the Barke Booster. Since the Check Valve is a one way Valve it makes a difference which way it goes.

On mine starting with the Brake Booster and going forward you will reach the Check Valve First. After that there is 2 smaller Nipples (some Models only have one small Nipple). Further forward is the Main line ending at the Vacuum Pump.

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84 300D, 82 Volvo 244Gl Diesel