bleeding problems

Hello, After changing fuel filters on our Yanmar 3GM30F, I tried to bleed the system by loosening the bleed screw on the secondary pump, and actuating the manual lever on the lifting pump. Nothing comes out. If I crank the engine, which also actuates the pump, it seems to work. It's a new pump...

I have a Catalina 309 with 1008 hours on an engine that has been well maintained. Repeated bleeding of the injectors has not allowed starting the engine. A mechanic has indicated that it is a simple problem (which he will not disclose). Does anyone else have knowledge of a simple explanation...