As previousl stated it could be one of the Valves in the Lift Pump not seating.

However, one of our members bought a New Style Bosch Hand Primer and it leaked. He went to the place where he bought it and exchanged it for anoter and that on also leaked.
He figured they had go a bad lot of Hand Primers so he went to another branch of the same Auto Part store and got another one that and that one was OK.

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84 300D, 82 Volvo 244Gl Diesel