Cooling System Does Not Build Pressure

Cooling system won't hold pressure. car does come up to temp. recently got one of the gimmick caps that have a temp sensor and im at 190 - 200 after a long drive, at the cap, but no pressure. I've replaced the thermostat a few time already, as its cheap. Problem didn't really stat happening until i bought a new radiator from the auto parts store.

Pressure tested system and cooling system holds pressure, minus the cap. I have tried 3 or 4 new caps at this point and it doesnt solve the problem.

The only thing i can think is maybe the plastic molded tank that the filler neck is on, from the auto parts store radiator might not be correct, and isnt allowing the cap to seal?

Only other thing i can think, and maybe it is or isnt possible, it that my car is overcooling? Do cars not build pressure if its overcooled, or does the temperature just not come up? My car does have spurts where on the dash gauge the temp is half a line below operating temp. If i drive with all the accessories on it comes up, but i dont know if thats overcooling or a bad radiator?


Any help is appreciated.

Just adding that the car hasnt overheated. Checked coolant bypass lines with temp gauge and their at tem

Aluminum heads and edelbrock victor water pump. was running the water pump at stock speed, but now i'm under-driving it. Didnt make much of a difference.

edit: Okay, I see the pressure buildup causes the system temp to rise by 45 degrees. That leads me to believe my cap isn't sealing correctly. Thanks for the information.

I'll see if i can warranty the radiator. I have 3 or 4 spare caps around the house that all do the same thing. When i pressure tested the system it was hard to get it to seal on my filler neck. If i jiggled or bumped the tester cap it would lose pressure. When it sealed it could hold 16 psi for 20 minutes.