overflow tank pressure

[Hemidog]

I bought one of these tanks to keep the car from spilling coolant, but the thing holds the pressure, so when I open the tap, the pressure is relieved
That cant be right? the radiator needs to relieve pressure right?

http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2D300102&N=700+115&autoview=sku

[67_Dodge_Charger]

Does it release the pressure after the engine and radiator have cooled down?  I would think that it would release the pressure/antifreeze back to the radiator.

Robert

[Hemidog]

it's not that kind of tank, it just catches the overflow, and you have to empty it once in a while...
usually the hose from the radiatorcap just releases the pressure on the asphalt, but this thing is airtight....

[suntech]

Hmmm
You have a radiator cap that holds a certain amount of pressure, right?
Then, when the pressure exeeds the pressure the cap holds, then it pops open, and what normally would just evacuate out the hose, will now "pump" up the catch tank, and since this one doesnt have an airbleed, i would think the rubber gasket in the top of the cap would seal enaugh, to hold some pressure. As soon as  you have a pressure below what the cap will hold, in the cooling system, the bottom seal will seal up in that direction, and the tank will be trapped pressure.
Rolf

[John_Kunkel]

With that type of overflow you need to have the radiator overflow plumbed to the bottom of the tank and the top vented to the atmosphere.

[Hemidog]

With that type of overflow you need to have the radiator overflow plumbed to the bottom of the tank and the top vented to the atmosphere.

ahh, off course! stupid me! 

thanks guys!