I want oil pressure tanks to place the prop in course pitch, low  RPM. if
I have an engine or oil pump loss.

Kevin,

 

Consider the following:

 

If the engine quits running but is still turning there should be enough oil pressure generated to pull the prop to coarse pitch.  Perhaps the question should be, "How low can the rpm go and still provide enough pressure to control the prop?"

 

If oil is lost from the engine and it is still powered it will soon seize and stop the prop - not as good as coarse pitch but better than the drag disk from a spinning prop at flat pitch.

 

I can't quite understand how the complex plumbing with check valves and a pressure tank could take over and keep the prop in coarse pitch if the only seriously dangerous condition is where there was no engine power, no oil and the engine was not seized but still turning, thus no control over the prop after it went to flat pitch.  Maybe an electric prop is the answer.