Re: 66 Galaxie Gas Cap causing excess tank pressure
I had the same problem on my Ranchero; the pressure got so great that it split the rubber fill hose. I read up on solutions, and many suggested creating a vent line from the top of the tank to the top of the filler hose. But I still didn't see why that would reduce the tank pressure; it just seemed to address the classic 'gas drip' down the fender. I decided to experiment with the cap (O'Reilly #6024) to see if there was a way to make it vented both ways. My understanding is the same as 1964fastback's, that the vent is one way 'in'. I completely removed the plastic vent mechanism and had a good look at what was behind it. I noticed two small rectangular holes to the outside of the sealing edge, and wondered if these could be vents. I set the cap upside down on my bench and poured a little water in the center of the cap. Water flowed out of the two vent holes, indicating vents to the outside.
It now works just as I'd hoped, vented in both directions, without holding pressure. Be aware that there is the chance of a small amount of fuel escaping if driven hard on twisty roads, but it's been 2 years of daily driving, with no gas down the side either.
And I suppose I need to include a disclaimer this day & age: I did this modification at my own risk, and you do it at yours as well.
1965 Ranchero Deluxe 66B
Factory 289 4-speed