Cracked heads or bad head gaskets that leak into the cooling system tend to build more pressure in the cooling system, not lower the pressure.

Are you sure you have the serpentine belt running around the water pump pulley the correct direction? I've seen the belt routed around the water pump the wrong direction and that causes all kinds of problems.

What does "thermostat opens and trickles out enough to get top tank of radiator hot,,cap on and I don't build pressure and it starts to over heat" mean?

How do you know the cooling system is not building pressure? Do you have a pressure gauge on the cooling system someplace?

How hot is it really getting? Do you have a known good temperature gauge on it?

Is the radiator plugged, or partially plugged? Are you loosing coolant? A system that holds a consistent 16 lbs of pressure with a pump that has no sign of a leak should be pretty sound. Gene