Overheating

My 04 truck has, occasionally, been overheating over the past 6 months and I think I have solved it.
The overflow tank was filling up when hot and not being sucked back into the system when the engine went cold.
Obviously there was an air leak !
I replaced both radiator pressure caps. No change.
After testing for blown head gasket and many other horrors, I cured the problem with an extra rubber washer from an old rad cap.
One of the new caps was not sealing properly ! If only I had tried that first !
There was no sign of any water leak but air was able to leak in enough to ruin a vacuum.
Lesson learned. Hope this can help someone else.

If the problem was on the cap on the pipe, it could well be corrosion on the seat that's causing it. A big seating tool or a wire brush in a drill can clean it up (best done off the vehicle though..)

Thanks for that PFM.
It was the one on the radiator which I solved with the extra washer.
I will take your advice if either of them fail again

The tank on the rad is plastic though, so best not go at it aggressively!

On the one inside (metal) I used a tap seat facing tool (being careful) to clean any corrosion and pitting of the sealing face. Next time I intend swapping it out for a new replacement.