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you have the radiator too full
Do you have a "dangling disc" radiator cap?

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rad too full will do it if you have a regular non recovery cap it will not be drawn back in. With the dangling disc (recovery) cap/overflow bottle you want it full to the brim and it will burp/cycle to remove as much air as possible. Check that before replacing the cap (& get the right one you need) recovery or non recovery & recovery is better as eliminating alot of the air slows rust and less air pockets give more uniform cooling (better/less hot spots)