Radiator expansion.

That is a good idea- I’ve had an expansion bottle on my 11B for many years. I’ve seen the overflow line attached to the drain valve on many Tractions- No idea why.

I had this problem on my car with the same stupid design of drain valve. I discovered it in 2012 on the way back from the Harrogate ICCCR. Hot day, traffic etc, finally getting going after the toll booths on the QE2 bridge, my son, in the front passenger seat announced it was raining. But only on his side. The solder around the header tank had burst as the breather was blocked. I believe the idea was that with this design, you could attach a tube to the pipe end and drain the radiator into a container rather than the cradle. But my experience was that when draining the coolant it would exit along/around the valve stem anyway.

But of course my only experience was probably with a blocked pipe so maybe if the pipe wasn’t blocked it would actually be effective. To be fair, I don’t really know if combining the drain valve and the breather makes it more likely for the breather to block than a separate pipe. I just suspect that it does because the valve stem gets in the way and is a side for any salts to collect. Chop the pipe.

Now that is a classy overflow tank. But being as we have French cars, I would think a bottle from a vintage cognac would be more fitting.