replaced water pump, Impeller blade completely dissolved into system.

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after replacing the Pump I flushed the entire system,Removed radiator and checked for flow,which is good now.thermostat has been replaced and tested also.Removed freeze plug and power flushed engine block, all hoses have passage including heater core.I still have a serious pressure and overheating problem after flushing with several agents about 10 or more times to get all the rust colored water out.even though there is no obvious detectable problem with the heads, no leaking or steam in the exhaust,or water in the oil.the motor still runs well.could there be blockage in the heads or breached head gaskets ? It seems like the pressure is building up in the out flow of the radiator hose going into the intake manifold.I checked the thermostat several times and is working fine.could there be blockage in the intake manifold or in the passage in the water ports in the heads.? could the head gaskets somehow be blown out or clogged with debris.? Is there a way of checking this before dismantling the engine for further inspection.... sincerly C.Davis , Det. Mi.