Is your sample porous? I have seen this with meso and micro porous materials. If you can be more specific about porosity we can discuss little further.

it is porous, but the pellet containing S (3-4%)

Dear Adam, you question have one error. You don’t absorb Hg vapour; you introduce liquid mercury inside the porosity of sample at high pressure, you made the mercury intrusion at room temperature and high pressure. The reaction of elemental sulphur with mercury is spontaneous and exothermic at room temperature. If you have elemental sulphur, no in other form of sulphur, probably you have chemical reaction. But only when the sulphur is in element form.

Be carefully whit samples whit sulphur when you degasified the samples to make mercury intrusion or nitrogen adsorption. The S whit the temperature and heat sublimate and collapse the electrovalves and the tubes inside the equipment. The solution is replacing the equipment.