The first thing I would do is to turn those coolers 90 degrees. With the with both inlet and outlet on the bottom you can have air trapped in the top of those tubes. then make sure that the front cooler line goes to your radiator first (or one of the bottom tubes if you are not using the radiator cooler) then a line from the top of the first cooler to the bottom of the second cooler and from the top of the second cooler to the rear fitting on the C4. That will eliminate any trapped air.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions