problem not solved

[mik]

filled rad and reservoir 3/4 full on my '69 124. ran engine with rad cap off upper hose did not collapse, did not overheat, placed rad cap on ran for awhile longer, hose did not collapse. shut off engine and walked away thinking problem solved. next morning upper hose was collapsed. when running both hoses warmed up nicely. Is it rad cap or upper hose or both?

[azruss]

you radiator cap has a 2 valves. first one opens to allow hot coolant to the overflow tank. this is usually the big spring and washer on the bottom of the cap. The 2nd opens to allow coolant from the overflow tank to enter the system as it cools. this is a small valve located inside the big spring. This valve is not opening so when you system cools, it forms a vaccuum, thus collapsing the hose. replace the cap.

[majicwrench]

Rad cap has to be able to let pressure out when it builds, and let pressure in when radiator cools. If hose springs back to shape when you take cap off, get a new cap.
Keith

[mik]

Thanks for the responses, and the detail on how the rad cap works. It helps to understand how and why it happens. I will get a new cap and hopefully problem will finally be solved. Mik

[BEEK]

A lever vent cap will usually not work; make sure the cap you buy is for a coolant return system. I recommend buying an OEM style cap, one of the vendors should have those available at a reasonable cost.