You can find wiring diagrams at http://www.gomog.com/allmorgan/44WiringDiagrams1998on.html

According to that the brake fluid level should be black and white. Black should be the earth (negative). I can't find any likely reference to black and green so given its location I think Max5 is correct and it is the brake fluid level switch.

I suspect that the cap should look something like this:

[Linked Image]

Peter

[Linked Image]