Yes, it could be moisture. Bleed the whole system.
Sounds like you are due for new rubber or stainless steel brake hoses all around.
Do you ever have any problems with the stop lights not working?
I have to replace the stop light switches on my 1973 Beetle every three years to keep the stop lights working in good order.
Do you know where I can get some stop light switches made in the U.K.? Thanks again!