That's a spare piece of fuel line, who knows what it's there for.
The fuel cutoff line is brown with a blue stripe, and there is a brown plastic line attached to the maine vacuum line from the pump (in front of the IP -- the IP (injection pump) is were the stop lever is) to the key switch.
If the brown line is broken, disconnected, or the rubber connector is off or cracked, you may not have enough vac to pull the shutoff down.
The shutoff solenoid can be bad too.
To check find the cutoff solenoid (metal canister with black rubber hose bit around an inch long and a brown plastic line with blue stripe attached). Pull the rubber connector off, push the stop lever down, and cover the fitting you pulled the hose off with your finger. Let go of the stop lever and see if it stays down. If not, the solenoid is bad, you need to get it replaced. If it does stay down, you have a vac problem. Check the brown line for vac by pulling the connector off the main line. Be careful not to break the plastic, ti's probably brittle. If you can put your finger over the fittgin and feel vacuum, there is something wrong with the line or keyswitch. If not, you have either no vac (and mo power brakes) or the fitting is plugged. YOu can clean it wil a small wire.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!