Cooling

Hi all
I have now ordered a new brake master cylinder so I hope this fixes the problem.
I have been doing some more work on the old Landy she is nearly ready for an MOT
I have noticed next to the oil filter just to the right is a rubber bung with a clip on it. I think it is part of the cooling system? it’s not leaking yet but has cracks in it anyone know what this is called? I have looked in the manual to no avail……
Cheers
Martin

Engine?