Firstly my apologies Kevin for hijacking your post.
Test result in.
Using my foot pump I attached it to the overflow pipe. Gave it a few pumps and noticed a few drips out of the top hose to rad joint and hence no pressure. Tightened the clamp! Tried again and got up to 10psi but it reduced to zero in less than 30 seconds. Unplugged pump and no pressure in the overflow hose. I then carefully undid the rad cap (note the system is cold) and air and fluid gurgled down the overflow hose.
Is it the top cap seal that is leaking? What seems to happening is I've pressurised the coolant system from the wrong side. I'm unsure how that could happen if the bottom seal is working correctly. There was definitely pressure when I release the cap so the bottom seal was doing something. Next test is to see if any fluid rises in the overflow when the engine is hot.
Just read Johnny's post, there's a vacuum release valve?
Hmm, I'm not twiddling anymore today as I'm driving to Brands tomorrow for a historic race meet and I don't want to prod the Gods!
Any thoughts?
Iain