This pic shows a checkball in the main inlet port.  Mine has no checkball there.  Was that a later addition or is it required.  I would think it would help maintain prime, but that depends on how other internal check valves operate.  Unfortunately, I don't have a detailed manual for the CA.  I have a pseudo exploded view of the WD, but it shows a suction tube instead and has no reference to a checkball there.  I also don't see a convenient attachment method.  I don't think it is threaded but I'll check closer when I remove it for the 3rd time.