Morris Minor Owners Club

Having re-read your posts this seems rather 'odd'.
Is the fluid level right up to the top? that would cause any m/c to leak a bit. If it still leaks out of the cap when the reservoir is half full then you should be able to sort it by adding a baffle in the cap, but I'd be surprised if the leaking from the cap could empty the entire m/c!!

You could try placing a piece of soft plastic sheet over the filler hole and then screwing the cap back on to see if the system then works properly but, the
plastic will cause a vacuum eventually unless you make a small hole in it.

I might have a go with the plastic sheet idea and see what happens, I can only assume the reservoir has drained through the cap as I can't find any other leaks and air had got in to the system. The reservior is about half full now, but it still leaks out under presure.
More experimentation is required I think.

Just an up date, I think I have solved the problem by fitting modified one way valve from a washer system into the inside of the cap. It lets air in but not fluid out.
So far so good