Depressed pressure cap

Claim 1

IN A SYSTEM OF THE CLASS DESCRIBED, A TANK HAVING A TOP WALL, A FILLER NECK MOUNTED IN AND PENETRATING SAID TOP WALL, SAID FILLER NECK BEING FORMED TO PROVIDE AN UPPER MOUTH, AN ANNULAR UPWARDLY-FACING SEAT BELOW SAID MOUTH, AND A TUBULAR EXTENSION BELOW SAID SEAT AND TERMINATING IN A LOWER MOUTH DISPOSED WITHIN SAID TANK AT A LEVEL SIGNIFICANTLY BELOW SAID TANK TOP WALL, SAID NECK BEING PROVIDED WITH AN OVERFLOW PORT BETWEEN SAID UPPER MOUTH AND SAID SEAT, AND SAID EXTENSION BEING PROVIDED WITH A BLEED PORT BETWEEN SAID LOWER MOUTH AND SAID TANK TOP WALL, AND A CLOSURE CAP FOR SAID FILLER NECK, SAID CAP INCLUDING A COVER MEMBER RETAININGLY ENGAGABLE WITH SAID NECK TO SPAN AND COVER SAID UPPER MOUTH, PORTED VALVE MEANS RECIPROCABLY SUPPORTED FROM SAID COVER MEMBER AND YIELDINGLY PRESSED INTO CLOSING ENGAGEMENT WITH SAID SEAT WHEN SAID COVER MEMBER IS SO ENGAGED, A SECOND VALVE MEANS CONSTRUCTED AND ARRANGED TO CLOSE THE PORT IN SAID PORTED VALVE MEANS AT TIMES BUT BIASED TO OPEN POSITION, AND A GASKET CENTRALLY SUPPORTED FROM SAID COVER MEMBER AND DISPOSED, WHEN SAID COVER MEMBER IS SO ENGAGED, WITHIN SAID EXTENSION AT A LEVEL BETWEEN SAID LOWER MOUTH AND SAID BLEED PORT AND IN PERIPHERAL ENGAGEMENT WITH SAID TUBULAR EXTENSION.