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Pumping Away

I have a 199M munchkin boiler piped primary secondary with indirect heater. Both pumps located downstream of the expansion tank which is on the supply side of the boiler. I have a pressure gauge on the supply right out of the boiler.
When I run the indirect, the gauge increases from 15 to 19 psi. However when I run the primary pump, the gauge decreases to around 10 psi. This makes no sense, since the ponpc at the expansion tank would lead me to believe the pressure can only be higher. The primary pump is a Taco 0011 and the indirect, which is undersized is a 0010.