New Dunlop gold seal tire on my 74 z1

well i took of the warn out continental conti blitz, and replaced with a dunlop gold seal, like the one that came stock on her. Alone frowm the ddeper thread groves its a really good and smooth ride. I opted to do the tire myself vs. bringing it to the bike shop for 50 + $$. It wasnt as hard as i thought it would be, the only difficult part was dealing with the rim locks. But the tire rides and feels really good on the bike. I got scared though, when i initially put air in it i heard the bead pop, then stopped cause it showed 40 psi. Then i came back about 30 mins later and took it off the center stand, and it was flatt. I put some more air in it to get it up to 40 psi, and rode it bout 8 miles or so with no problems. I dunno why it went flat though????? any ideas