Coolant system vacuum during cooldown

[HoldOn SV]

2013 L77 41K miles. Seemed more like an engine question, so posting here: I noticed the stock two-piece upper radiator hose collapses overnight. No stored codes in pcm. Coolant is full in radiator and on dipstick in reservoir. No apparent cooling issues, or leaks, and no coolant visible in crankcase. I'm curious why the system would create enough vacuum during cooldown to collapse the hose. Any thoughts?

[cerbomark]

I changed my radiator cap and it stopped doing that, at least the last time I checked. Caps are pretty cheap at dealer.

[HoldOn SV]

Thanks very much for your feedback cerbomark! I'll change the cap and post an update. Cheers.

[HoldOn SV]

Replacing the radiator cap resolved this issue for me. Cheers.

[cerbomark]

Great! Now we know, LOL.

[HoldOn SV]

Yep. The o-ring seal that allows the cap to function at 140kPa/20PSI was faulty.