I have a 1985 1.9 litre Westfalia. I have been having problems with the cooling system. While driving, the coolant expands and overflows to the point where the van begins to overheat. I just performed a leakdown test and only #3 came out in the low end of low high end of moderate around 35 percent. When I run the engine with the main coolant bottle open I can see gas and foam as I rev the engine. I think I may have a leaky head gasket but How do I find out which side or do a definitive test.