[ngoma]

The nut does not seal the oil. The nut merely holds the vacuum pump tight to the side of the cylinder head. The large o-ring in the groove of the vacuum pump mating surface seals the oil.

Suggest you fit a new o-ring seal. Clean and make sure the mating surfaces on the cylinder head and the o-ring groove on the vacuum pump are clean and free of scratches, dents, chips, or burrs that could allow seepage.

Take care when fitting the o-ring to not tear it or let it fall out of place. You can use thick grease to help keep it in place in the groove when reinstalling. And tighten the nuts evenly, bit by bit so the clamping pressure is evenly distributed.