Header Tank level

[icosahedron]

OK, I wasn't aware of the different types of blue caps. Mine all have a one-way valve and a safety release valve. One with a pressure regulator should cause the coolant level in the replenish tank to rise and fall with engine temperature in the same way as in my expansion tank with trapped air then. Is that correct?

[Simon Baxter]

Have a read of the header tank cap paragraph.
https://brickwerks.co.uk/blog/vw-t3-t25 ... -bleeding/

I have no idea why VW put min and max levels on the header tank, just confuses the matter.

[Alfredo]

Expansion tank, the one behind the number plate flap .
It’s connected to the spout on the header tank via a short rubber hose. The coolant level in this goes up and down with engine temp, the only time to set the level of this is when the engine is clap cold as it is the only constant you can get

What should the expansion tank level be at when it is cold?

[icosahedron]

I tested one of my blue caps last night using an airbrush regulator, and lo and behold, contrary to my expectation, the valve opened at around 1 Bar. This changes everything: all this time I've been barking up the wrong tree by thinking it was a safety release valve that would only open at well above 1 Bar. I'm devastated, what a waste of time and effort. I need a holiday now.

My arrangement must've lowered pressurisation to the extent that it doesn't hurt the lower-quality expansion tanks. No harm done fortunately and easy to set right. I don't use the vehicle on a daily basis anymore and an expansion tank should last a while.
  

[Simon Baxter]

Expansion tank, the one behind the number plate flap .
It’s connected to the spout on the header tank via a short rubber hose. The coolant level in this goes up and down with engine temp, the only time to set the level of this is when the engine is clap cold as it is the only constant you can get

What should the expansion tank level be at when it is cold?

Max ideally but to be fair, doesnt matter if they are over filled, they'll just spit out what they don't want and it'll find it's own happy place.

[Alfredo]

Max ideally but to be fair, doesnt matter if they are over filled, they'll just spit out what they don't want and it'll find it's own happy place.

Thanks

[icosahedron]

I

Or simply your black tank cap was duff and now it's been replaced with one that works properly then the cooling system now work properly.

You nailed it right there. A malfunctioning cap that always left air trapped in the expansion tank right from the start and myself accepting that as the norm. Compression of trapped air provided enough pressurisation to ensure trouble-free operation of the cooling system, and the presence of the MIN level indicator backed up my understanding of how it worked.

All this time I was blaming a low-quality blue cap one-way valve for allowing trapped air to escape. This is the reason why I tried another one-way valve in series, fortunately with trapped air in the system that could be compressed.