Coolant flush disaster Options

[The_Chazzman]

Hey, all. I'm trying to flush out my coolant system on my ST204 so I can get all the green junk the previous owner filled it with and replace it with that beautiful red juice. I drained as much of the green stuff as I could, pulled out the thermostat, re-installed the water outlet and proceeded to fill the rad with water. I could hear it making its way through everything until it pretty much all wound up pouring out of the outlet. I would assume this is because there is no stat/gasket in there but, as I can't flush it with them present, what am I supposed to do? Anyone else have this issue? Thanks in advance.