I have a motorhome with a 5.7 350. How do I clear a coolant blockage?

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There is a tick/click noise like a mechanism isn't engaging; coolant is overflowing into the cab thru the floor. It overheats after 10-15 min at idle. I believe it may be a heater line blockage or a radiator blockage, just not sure where to start looking. The radiator hose coming off radiator is pressurized after motor has been turned off also. Any tips would be greatly appreciated. T