I'm looking forward to know what you have. What is puzzling is that according to the spare parts catalogue Longc and Deauv there should be a press cap on the rad and blanking cap on the overflow tank, if you look at the drawings. I also checked the spare parts catalogue for the Quattroporte III/ old Ghibli which has a very similar system. Same radiator and identical FIM overflow tank and according to these cars there should be a 0,9 bar press cap on the rad and a 0,5 bar press cap on the overflow tank! Go figure that!
I've found on a few cars the soldered stub tube for the hose that connects the rad cap overflow to its catch tank is cracked underneath at the solder joint, so while it doesn't leak water (or not much), it DOES suck air back when the engine cools off. I've also found cracked and perforated hoses in the same area. The cracks tends to 'use' water and are very difficult to trace. On U.S Panteras, we also found that U.S radiator caps are a little shorter than Euro caps, so even brand new ones tend to leak around the cap. And even when they don't leak, the short length changes the pressure rating of the cap. For us, the 'fix' was to desolder the European rad-tank neck and reattach a U.S radiator neck. You are using a Euro-made cap, right? The Euro cap I have (from Charlie McCall's GT-5) is a four-ear model stamped "Serrare Bologna 0.9 bar"
My Deauville has a pressurized overflow tank, so it seems correct to have two pressure caps. It's an odd system; the lower cooling tube/hose to the rad is directly connected to the tank while the rad is connected to the tank through its pressure cap overflow. My tank was always empty too, that's because the tank is smaller than complete system volume differential. I have changed my system; plugged the lower connection and made the tank unpressurized (connected only by radcap overflow), but I haven't found a tank to fit yet (ie big enough).
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