Question about radiator vent hose

[moto9]

Finally got my 08 EC 300 and I noticed that the radiator vent hose was folded over and zip tied closed...I thought this was done to keep it from leaking during shipping so I cut the zip tie off.
The other day the bike steamed over in a nasty section and the catch can didn't seem to be working, so I took a closer look at the plumbing and it didn't make sence how it was routed.
Every catch can I have seen routes the radiator vent tube to the bottom of the can and the top of the can vents to the outside air.
So, as the bike steams over thru the vent tube the fluid is dumped into the can...as the bike cools the fluid is sucked back back into the radiator.
The current plumbing is a hose off of each radiator routed to the catch can with no vent to air...I don't see how this can work properly.
Any insite????

 

[PEB]

It is an expansion tank, works great on my bike others may feel differently.

 

[gasgasman]

When the catch tank is empty, it has atmospheric pressure in it.
As the coolant fills the catch tank, there's still an open area of air in the catch tank.
As the coolant contracts in the radiators the coolant is sucked out of the catch tank.

The vent hose purges whenever the pressures exceeds the radiator cap rating.
The "pressure" in the catch tank is lower than the radiator cap rating, so the catch tank fills up first.

The coolant level needs to be about a 1/2 inch below the sealing surface of the radiator cap.
That will prevent "boil overs".

Isn't physics fun?

 

[jefgil123]

Yes the GasGas plastic "expansion tank" should be on every dirt bike,
as the coolant gets hot, it expands a suprising amount, and there
is nowhere for it to go but out the radiator cap and to the catch
tank, (if there even is one)
The expansion tank prevents this by giving the coolant room to
"grow" and still remain in the system, doing good.

I don't know why radiator top tanks aren't a lot bigger, to allow this....