Reply to "cooling system, Longchamp/ Deauville,"

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"