L98 coolant overflow problem

[Koupq]

Just have a question as to what this little flap thing is on the coolant overflow bottle?

Car overheated heading down the highway yesterday, never had an issue with the cooling system prior.
But earlier that day I was trying to replace headlight globe, (caprice model) and reading the manual it said to pull the neck out of the overflow bottle to get into there, (which doesn’t work to change the headlight for the caprices anyways)

So I’m thinking I may have not pushed it back into the overflow properly and that’s why it overheated?

Am trying to flush the system now, and the radiator and hoses hold water fine, but when trying to flush the overflow, it pisses out this little flap bit. Is that meant to have water come out of there?

Have pulled the hose off from near the filler cap and so that’s at the lowest point now, but it won’t flow out of the hose, instead comes out this flap?
Cheers

 

[Koupq]

Even with the flap pushed shut, there’s still that little gap at the bottom of it which is where it flows out of

 

[Asmodai78]

yeah the would be the overflow for the over flow bottle if it didn't have that it could over pressurize and break open, that flap would be there to stop dirt falling into it, as for your overheating problem replace your radiator cap and thermostat and that should fix your issue