OK Gordon. I was not picking on you. I just did not understand why all the concern over a 7 C temperature change.

You have an interesting problem there. It only leaks when cold below freezing. I have no ideas on the cause for that. I agree that if it leaked when the valves are shut off, that it is probably a bag leak but I also have never worked on a 4 bag system so I do not understand the plumbing involved.

On the soapy water idea, How about using some windshield solvent and soap mixed together? I have never tried that. Maybe that mix will not bubble.