I forget what this means, but my lower radiator hose keeps deflating or suctioning itself flat. Headgasket problem?

 

never heard of this one, im interested too

 

Is this when it's cooling down or does it do it when it's running?

When cooling down...
Check the smallest disk-seal in the radiator cap. That disk should pull down very easily. What that's for is so the system can suck coolant back in from the reservoir at the slightest vacuum. Then look for blockage in the little hose going over to the reservoir.

If it does this when running, I'm not sure. Maybe a really plugged-up radiator?

 

It only happens when it's cooled down or cooling down, when the car is off. I'll take your advice

 

If there's crud in those springs & seals of the cap, it's not even worth cleaning. New cap is pretty cheap.

 

7 bucks for a new cap

 

Replaced the greddy cap with a $10 korean cap, and it fixed the problem! thanks guys!