I had the same issue when I recently serviced a 2GM20F. I had removed the bottom of the racor bowl to clean out debris. When I came to bleed it I just couldn't get fuel to the injector pump. It turned out that I had disturbed the o ring seal of the racor bowl and it hadn't seated properly and was therefore drawing in air. I reseated the bowl and manually pumped the fuel through to injector pump and she fired right up and ran beautifully.