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This used to happen to my C3 all the time. You also need to check that the radiator cap reflow valve (the little disk in the centre) stills works and that your overflow hose is not blocked.
What should happen is that when the cooling system gets hot, excess water is fed through the radiator cap to the overflow tank. When it all cools and the radiator gets negative pressure, the raditor cap reflow valve should open and that overflowed water is pulled back into the cooling system.
Your collapsing hoses are an indication that the cooling system is at negative pressure (so the outside pressure squeezes your hose flat).
In practice, up until your hose starts leaking, none of this is a problem since when the system pressurises at temperature the hose expands again.
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15-'16-'17-'18- '19
Re: Upper radiator hose found FLAT! (indypace78)
It's definitely the cap......get a new one .....same part number as the old one. The new one is 18 psi as compared to the old one which is 15 psi. I have a 98 coupe and had the same problem. I've had the new cap for over a year, no problem. Part number 25713160..... remember, the part numbers are the same, the psi is higher with the new cap.