Fuel pump losing prime

[J Lee]

my 79 blazer has had this problem ever since I bought it. It had been sitting for a while when I bought it but you could fill the carb bowls with gas a couple times and it would prime up and run but would lose prime if allowed to sit overnight. I assumed this was due to the junk Edelbrock carb that was on it and the fuel pump going bad. I have now changed the carb to a new 600cfm Holley and replaced the fuel pump but the problem is still there. once it is primed in the manner described above it is fine and will run and will crank again after sitting a short period but the pump is still loseing prime if it sits overnight. All the lines are tight and it is not leaking gas anywhere so I can't figure out what could be causing it now. any help would be greatly appreciated.

Jeff Lee