oh, I didn't realize he meant air was in the upper hose, I thought he meant it was just deflated when cold from the coolant contracting. Anyway, make sure the expansion tank is full. My guess is that it is low which is how air go sucked in

 

delated? did you man collapsed? if so thats bad to.

all hoses should be full when cold thats why the rad cap is suppose to be the highest point so you can fill the the highest point. also make sure the over flow tank has enough in it. it should be at least to the min mark when cold. I usually fill overflow tanks on any vehicle to halfway between the min and max marks when cold.

 

7pmaybe newer cars don't do this as much but on older cars the upper hose would have coolant sucked out of it when the engine would cool off. It wouldn't get filled with air, but it would look squished. If you figure that the pressure required to open the seal in the cap takes (depending on vehicle) between 15-18lbs of force to let excess coolant into the overflow tank. When the engine is cooling off once the pressure gets below 15-18lbs the seal closes, but there is still pressure that needs to dissipate inside the system. Since it can't just escape it is going to try to contract things, like rubber hoses, to make the volume of the system smaller, and thus equalize the pressure. Since the system is sealed, so air can't get in, a vacuum is created inside the system.

Now, I've never really paid any attention to this when opening the radiator cap on newer cars, but on older ones the seal would be sucked down onto the radiator, and when you'd open it you'd hear air rush in from the vacuum that was created inside of the cooling system

 

actually there is a little spring loaded valve on the bottum of the cap to let coolant back in that takes very very little vacuum(contraction of the coolant) to open. I take it you have no idea how a rad cap works

 

I do know how radiator caps work. I'm just saying that after growing up in a family of mechanics that operated a dirt tracks racing team, working on tons of old cars, most of them would have the upper hose deflate when the engine got cold.

 

maybe racing engines are different, but on modern street cars blind monkey is right, coolant is drawn back in and the hoses should not compress, go look at your truck when its cold, i bet the hoses are round, as designed.

 

The cooling systems in race engines are definitely more heavy duty than normal engines. It's not just race engines that I saw do that though, my old plymouth with a 318 does it as does my brothers dart 318, my grandfathers old chrysler with a 383, and even on old chevy small blocks. I used to have mine go flat when it was cold occasionally. I changed the radiator cap and it stopped doing that. I went out in my parents garage today and found that old cap. The small spring on it was corroded and stuck which was keeping the vacuum from "escaping". I think it may also have something to do with the strength of the hoses after 30-40 years of use on those old engines doing that also. Over the summer I replaced the upper hose on my old plymouth and the new one doesn't collapse when cold like the old one did.

Regardless of that, for the OP, make sure your expansion tank has the appropriate amount of coolant in it, make sure your radiator cap isn't stuck or anything, and make sure your fan(s) are working