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I have gas caps that leak when I go flying. This is evident by the fuel streaks down the top of wings from the filler necks after I land. I suspect this is from the position of the filler neck & fuel caps is steep turns. High wing tank leaks in a turn. I also noticed that the right fuel tank level was always higher than the left after going flying.
The tanks would start out level if left in the hangar over night. I went to dip the tanks after flying and heard a "whoosh" upon opening the right fuel tank...."a vacuum condition". I tested these tanks for fuel flow during construction and no problems. I can drain the tank by selecting the right tank, it will drain but takes much longer than the left tank to do so. For now I replaced the cap with a spare I know works off my CH601HD header tank. My CH601HD uses a CESSNA vented capP/N C156003-0101 or Aircraft Spruce #06-00985.
This is a vented cap with a red rubber anti-leak check valve on the inside of the cap. I removed the rubber anti-leak part so I can pump fuel from the leading edge tanks into the header tank without over pressuring it. To solve the messy gas leak stains issue on my CH750 I will install this same vented cap with the red rubber check valve removed. This will prevent the fuel tank in the wing of the CH750 from building pressure as this fuel pressure could possibly build to the point of overpowering the 3 psi Rotax 912 carb floats limit or even bulging the wing skins while sitting in the hot sun. Just like how a red plastic gas can goes sitting in the sun with gas it it. These CESSNA fuel caps dont leak with the rubber anti leak check valve removed, it solved the constant gas smell in my CH601HD cockpit issue.
The picture shows the right wing fuel level lower than the left, the Dynon D180 is calibrated accurately.