MB OE Parts are guaranteed for a year. I'd talk to the dealer you ordered them from and tell them the cap must have been faulty or the tank was defective...either way they should replace them for free if it has only been 6 months. I agree that if you had a headgasket leak the pressure would disappate if the cap was working correctly.

__________________
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz