Have not had a W123 dash apart, but the mechanical gauges used on pre 85's can and do get stress cracks in the bourdon tube. Sounds like this is your leak source. Might try to get a better one from a car in a junk yard. Be careful to use two wrenches when unhooking the gauge from the line and when reconnecting in your car. Also, do not start the engine with the line disconnected from the gauge!!!

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150