Top up tank empty after 10 mile round trip

[Mocki]

The system is very simple....
The headertank ( the one in the engine bay ) should always be full to the brim, and the top up tank ( behind the number plate flap) should be full to the max mark when cold. When the engine is up to normal running temp the coolant expands and is pushed into the top up tank, when the engine cools down the coolant is sucked back into the header tank

So if it is not sucking back, it hasn’t retained the vacuum or the valve in the cap is not working .
Not retaining the vacuum means a leak , maybe only in the pipe between the header tank cap and the top up tank( porous pipe), maybe in the header tank ( level sender or hairline crack ) or possibly another pipe junction not sealed .

Not easy to find, hot coolant evaporated before you can see it has escaped , maybe coolant not even escaping .

But one thing is for sure, only change one thing at a time , starting with the cheapest / easiest.

[Michael4]

But one thing is for sure, only change one thing at a time , starting with the cheapest / easiest.

Thanks for very sensible advice.

I guess the first thing is cap and then cap to top up tank pipe.

I've found the 'quack test' video as a starter, need to grow a beard first!

[Michael4]

Cap wouldn't quack unless I fiddled with the spring using a small screwdriver and attached it to a tyre pump. Then it would quack twice and stick. Another on its way...

It was only 18 months old.

[Mocki]

RIP cap!