I had the same experience. Improperly manufactured and I was never impressed with Hartwig’s customer service. I used the caps for a couple years. Troubleshot vents, doubled up gaskets, replaced caps on mains and aux tanks, removed chains, crush tabs, sprinkled fairy dust and poked voodoo doll etc etc. Had a couple flights where I lost 20-30 gallons. Glad I had EI fuel senders. Needless to say, I have given up on them and it feels glorious (and safe). I now use the old original gold cap monarch caps (I got on eBay) on the mains and they NEVER leak a drop and fuel comes out of the tanks evenly. I replaced my improperly manufactured monarch aux caps with vented Cessna caps and they work wonderfully as well. No leaks in over a year.

Before I was familiar with monarch caps, I remember flying to the lower 48 from Big Lake and stopping in Watson Lake for fuel. There I met a fella and his wife going north to AK and he was red in the face mad as he was refueling. Turned out his caps where leaking and he almost ran out of fuel north through trench. Scary.

I’ll have some new shiny monarch caps for sale soon for the fans. Maybe I just have bad luck.