An 86 SDL doesn't have a separate primer pump. Priming is done by the lift pump with the help of the starter and usually costs a battery

Follow the fuel path from the metal lines along the left inner fender to the small fuel filter to the fuel thermostat, up behing the oil filter to the fuel heater back to the fuel thermostat to the lift pump, then the plastic line to the big fuel filter, around the big bolt sticking down into the big fuel filter then back to the injection pump, then examine the where the injection lines connect to the injection pump, next check where the injection lines attach to the injectors as well as the return hoses that link the injectors. Work from back to front then to the fitting by the big fuel filter where the injector return line is joined by the injection pump return line then goes to a metal line on the inner fender.

Start by cleaning everything described that's not presently clean and dry. I use brake parts cleaner. Observe for a couple of days what gets wet with fuel.

Sixto
87 300D