Robbie, you wrote a caveat to explain what you meant by an unstable
"valve." However I think you are still laboring under the misconception
that someone at the factory occasionally placed a solid, airtight
cardboard cover into the Ampico valve's top seat and shellacked it in.
>[ Sorry, I used a poor expression. The valve is stable but the
>[ pneumatics it controls aren't. If only one bleed is used, such
>[ as at the valve seat, then equal suction is applied to both the
>[ reroll pneumatic and to the shutoff pneumatic. Therefore the
>[ bleed aperture is a compromise, and the one or the other pneumatic
>[ suffers. -- Robbie
Actually, your comment or its caveat doesn't apply in the Ampico B
case, anyway. It's fully stable and will never float between bleeds
or fail to seat tightly because the cardboard bleed hole isn't really
a "bleed hole." That was D.L. Bullock's implication, that some
cardboards didn't even have holes. If either were the case, it
wouldn't work at all. And if the actual factory hole in the card
was close to a bleed hole size, the system would not work either.
It's larger than a bleed. If the factory in some cases just pressed
in a cardboard blank, the leakage around the card would be plenty, too.
It should be obvious that something is wrong with the "unstable valve
system," theory here, because "C" valve is used only as the vacuum
supply for "D" valve. If C valve was inherently unstable, it would be
so whether it was controlling a pneumatic directly or through another
valve. Rest assured that the system never gives a moments' trouble.
You are referring to a pressure divider in which two equally sized
bleeds (or bleeds relatively close to the same cross sectional area)
will partition the pump pressure accordingly. And if the outside valve
hole is actually closed off airtight, there would be no valve action at
all. The poppet would float, not having enough air pressure above it
to seal it down. That doesn't happen here.