Engine Oil Leaking problems of the 2004 Nissan Frontier

Truck total mileage is only 55,000 as it is not used very much. 3 issues occurred approx. At 49,000miles. 1) engine started to miss. Upon checking spark plugs it was discovered that the plug cavities in the head were full of oil. The valve cover gasket seals had leaked and the oil filled spark plug holes caused it to miss. This is not covered under warranty so repairs were done at my cost. Had to replace all of the top end valve cover gasket and spark plug hole seals as well as the cam end seals. 2) one day I got into the truck and when I started it up and pressed the brakes the pedal went to the floor. Upon checking the brake master cylinder reservoir it was empty. Found that the rear axle brake line had rusted through and had burst spraying brake fluid all over the spare tire. I have never seen a brake line develop a hole like this. Not covered under warranty and a new brake line and master cylinder was replaced at my own cost . The system had to be bled out and flushed at all 4 wheels. I was very lucky that this did not occur while driving as a crash would have resulted. 3) currently having an issue with the "check engine light" coming on. The code says that it is a emission issue. With the computer code reader I am able to cancel the code and it stays off for a while but comes back on days later. The code is a leaking gas cap but I can detect no leaks and have changed out the cap with an oem cap with same results. Truck runs okay so I have not dug any deeper on this and it is still occurring. 4) while under the truck fixing the brake line issue I discovered that the drive shaft was completely covered with rust. It was never painted from the factory. I had to remove it to kill the rust and paint it with a good coat of black heat proof paint. Corrosion seems to be an issue with these Nissan vehicles.