Hi Stewart11 Right offhand, one of the few things I can think of is that you may have the fuel lines backwards on the carburetor. Make sure ALL fuel lines are correctly routed and that the fuel filter is laying on the bottom of the fuel tank. Many blowers use a remote primer bulb assembly, but many Echos now utilize the Rotary barrel carburetor with primer base assembly and bulb on top.
I have had that primer base go bad, and ultimately it was bad check valves or a restriction in the assembly itself. I see you have replaced yours already though. Symptoms you have stated indicate that the primer base assembly is not functioning properly new or not.
Did you SOAK or use any real harsh cleaners on the carb when you rebuilt it? IF so, you may have damaged it. Small internal check valves cannot take the harsh cleaners, nor can they stand up to high p.s.i. compressed air.