I recently rebuilt the cooling system on my 85 v12 so can sympathise. I would be inclined to lean towards a crack in the bottom hose or a leaky cross bar. Water pump is quite unlikely without any obvious noise.


If you do remove the crossbar remember exactly how you took it out and exactly which way the hose clips were facing (the same goes for all the hoses). It is a very very tight fit and quite fiddly to do and get all three hoses back on. Soak them in hot water and don't spare the lube!


If you find a cracked hose, you might want to invest the time to do them all - normally the heater core hoses by the firewall fail first (from my experience) in a spectacular fountain. The bottom hose is a pig, it will be oily and you will get cut - allow at least an hour!

 

UPDATE: Hey guys have been away on business and have not been able to tackle the job yet. All your inputs are FANTASIC! I will be able to detect issues when I work on the car. I will let you all know what the results are.