Pushing Water out

[sticksave]

Recently I started to get a low coolant light on my 2001 X5 4.4. When I checked it wasn't really down much so it just seemed like it was pushing a little out here and there. Previously I had taken it in under warranty for a similar problem and they did something with the resevoir tank and it was fine for quite awhile. I did notice that one of the flat square rubber seal rings was tweaked when I added a bit of fluid so I bought a new BMW cap and when I put it on the thing leaked like a sieve. The new cap had real O-rings instead of square rubber seals so it's possible there has been a design change. I have put the old cap back on and had intermittent problem but the last few days it tightened down better and seems to be fine again. Do the resevoir tanks go bad on these things? I should also note that the temperature never goes up and it only has spit water when you turn it off and stop. I drove all the way back from Tahoe at full speed and it didn't loose any fluids. Any thoughts out there?