I'd like to pressurize the heater portion of my coolant system with a coolant system pressure tester. I actually want to pressurize WVO coolant lines, which are tapped into the heater loop, to then bleed are air out of high points. I'd like to do this when the system is cold but then the thermostat will be closed. Without the thermostat open I don't think I can pressurize the heater loop. Any suggestions?

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'85 300td Frybrid WVO Kit