The W126 cars have internal vent valves at the top of the fuel tank that control venting to the charcoal canister. Over time these valves jam in the closed position due to deposits from the fuel or condensation in the tank which prevents the proper venting of the tank. On a warm day the pressure can get quite high. To my knowledge there is no way to remedy this short of replacing the tank or somehow adding a vent line to the tank and installing the external vacuum/pressure valve. I had my 380SE tank professionally cleaned with no resolution of the overpressure issue. No bad effects to the car yet. Mark