Rex,
Out of curiosity, I wonder if these bleed orifices have check valves to control the vacuum at which they bleed. I'm thinking that if the orifice is too large and not controlled, the pump might just pull in air and not pump oil. Analogy would be a two stroke motor with bad crank seals - - it won't start because the upstroke of the piston (vacuum pump) pulls in air thru the seals instead of fuel thru the carb. At high RPM where the pump is more efficient, the opposite might happen and the orifice might not be sufficient to prevent "stalling."
I wonder how many hours/dollars went into getting that right!
Tom