It shouldn't be THAT hard to put the cap on. It's possible that's the wrong cap. After my experience last month trying to find the right cap for my new radiator I wouldn't even be slightly surprised. But, if you're sure that the coolant expansion tank hose is clear all the way through I'd try a different cap.

StockRaider is correct degraded hoses will cause this, too, but IMHO with only 6K miles its extremely unlikey. The upper and lower hoses on my Sport were on for several hundred thousand miles and even then I never had problems with them collapsing. However, I kept them clean. If they get oil soaked all bets are off.

Edward