Mono jet won't start without priming

[retro4now]

Hi
I'm new to this forum and am finding it very useful.
I just bought a 1971 GMC step side with a later model 250 six.
It had been sitting quite awhile, so I had the tank professionally flushed and cleaned, changed the fuel lines, new sending unit, new fuel filters, and rebuilt the carb.
It's a one barrel MonoJet. It runs fine except for one thing.... When it sits over night it won't start without priming with some gas, no matter how much I crank it or pump it. Then it starts right up and every time after that until the next day.

Any ideas where I should start looking. I haven't had a chance to pull the top off yet first thing and check the float bowl. Seems like it might be leaking down from somewhere, but I don't know where that might be.

Thanks
Gerry
Albuquerque, NM
'71 GMC C1500