1989 Toyota Pickup Coolant back pressure in motor

I have a problem with coolant being forced out of the engine and out through the radiator. It seems that the coolsant is not getting back into the top of the block, and is being forced into the overflow bottle instead. I thought I had a thermostat problem, but it's fine. I also flushed out the block and the radiator, finding no blockages. The truck doen't have to warm up to get this pressure, just has to be running. What am I overlooking in my diagnosis, and what could the problem be?

Thanx, Rick