brown foam in expansion tank!

just been for a quik blast in the golf, got back 5mins later, opened the expansion tank lid and there was a very dark brown foam/bubbles…whats this??? because iv got a gt lump mated to a 1.1 radiator (built in tank) im worried that iv plumbed it in wrong or sumthing else is wrong with the engine or plumbing.  :dontknow:

It?s brown because it?s full of rust. Was it foaming after you took it off but not before?

The cap keeps it under pressure and raises the boiling point. When you remove the cap the boiling point is lowered and it err? boils?.. Antifreeze also raises the boiling point, do you have some in it?

If it is sludged up with rust I?d flush it out with some stuff you can get at any motor factors and stuff the hose pipe in it until it runs clear (with a cold engine of course)