Coolant level very high

[Alfredo]

I have suffered from coolant loss for the past year. Constantly needing to fill up the overflow tank behind the number plate.
My garage spotted a leak from a pipe during the last visit to them a couple of weeks ago and put a new tight jubilee clip on.

Not used since then.
Last night we drove 130 motorway speed miles. Before leaving the overflow was full to the max line.
When I checked again this morning the coolant was so high in the overflow tank that it poured out when I removed the cap.
I put the cap back on leaving the coolant well above max, at the level of the bottom of the cap hole.
Today we drove 350 miles at 55-70mph
The coolant level behind the number plate is the same, well above max to the bottom of the cap hole.
The temp guage has been at just above centre all the time.

So on the bright side my leak is fixed almost 500 miles without needing to top up, but can anyone explain why the level rose and isn't settling down to the max line please?

[Aidan]

faulty darlek cap perhaps, allows expansion into the top up tank but isn't working under vacuum conditions in reverse

[Rik]

Is the header tank fully filled?

[clift_d]

Aside from a faulty dalek cap, as Aidan has mentioned already, the other thing that can lead to transfer of fluid to the overflow tank is pressurisation of the header tank. If this is happening then it's an indicator of a possible head gasket issue allowing combustion gases to get into the coolant, which are then collecting in the header tank and displacing coolant to the overflow tank. As well as swapping out the dalek cap, it might be an idea to ask your friendly garage to do a "sniff" test for you.