> The old hose definitely had holes in it; I tested by partially submersing
>> it in water.  The new hose is airtight - and clear - and I saw how it
>> sucked fluid up from the overflow tank when the engine started.
>
>
> That last part seems backwards.  When the engine is started and is getting
> warmer, the expansion of the coolant in the system should force liquid or
> gases, whatever is at the top, out of the pressure tank and toward the
> overflow tank.   When the system is cooling down the coolant should be
> sucked from the overflow tank.
>
> Have you tested your pressure cap?
>

When I rev the engine, the pressure tank always sucks fluid down - the
reverse when the revs go down (barfing coolant onto the exhaust - mmm).

I agree with what you are saying when the system has heated up, but I had
just turned on the engine and went to check and, thanks to the clear hose,
I saw it had drawn fluid up.