hello guys,my 01 nissan patrol y61 with zd30 engine have continuos bubble in reservoir tank,my coolant in expansion tank look like it being suck in to the radiator but my reservoir overflow tank was overfull due to excessive pressure being push from my coolant system..could it be a cracked head?ive just top overhaul my engine..could it be my head gasket?or any other internal cracked or leak?anyone experience it before?