<< I read somewhere that you HAVE to have something in the top of the smog pump, or it will keep pumping into the manifolds and this is NOT good. Some kind of valve? True? What goes in this hole? Can I buy one? Can I run the engine without the belt attached to the pump? ... Danny >>
What goes in that hole is already there - it's a pressed-in pressure relief valve, to protect the pump in the unlikely event that the diverter valve sticks closed. The more common failure mode for the diverter valve is that it sticks open, pumping air into the manifold tubes all the time - that happens when the rubber diaphragm inside the diverter valve fails (and they all do eventually) and the valve then defaults by design to the open position, also causing a vacuum leak through the vacuum signal hose.