I get the feeling my radiator cap is weak. When I obtained my 88, there was no coolant in the upper hose - we refilled it and it holds coolant without an issue. I finally was able to take the 88 for a drive and when I returned, it was peeing coolant on the floor. A check after it cooled down and there's no coolant in the upper hose again. I don't see any other obvious leaks so I'm thinking that the cap is weak.
That said, what is the pressure for the cap? I hate to order a $8 cap from an LR-specific shop then pay $15 in shipping when I can buy a $10 cap locally - that is unless, like my VWs, there is something special and mythical about using the OE part.