Rob,

I am not sure how you got it to build pressure into the coolant by just turning it over for the test. You did have all the plugs pulled, right?

I would more suspect the radiator. I would guess that the WP is pushing coolant into the radiator but it is not flowing through it. So, it either comes out of the neck (if the cap is off) or into the overflow tank. If the rest of the system is functional, it may be flowing enough to cool the engine but the flow through the radiator is clogged enough to push coolant out, too.

Good Luck,
Michael