If it is from the vaccum pump. All it does is takes the air removed from the system by the vacuum pump and feeds it into the intake system. I do not have a 115 diesel.

You can ignore it if it traces back to the vacuum pump.

A problem occurs if the diaphragm fails in the vacuum pump and oil feeds up that line. But there appears to be no oil so save your money. The later vacuum pumps discharged the air into the crankcase.

Again I am far from positive not owning one of this vintage. Just see if it starts at the vacuum pump on the front of the engine block. It is just a filter on that line I expect if the vacuum system is operational. That being true if you can turn the engine off. Or have brakes.