Gas tank venting on '89 F-150 (dual tanks)

Hello,
Another question on my "new" 1989 F-150. I bought this truck last week in Colorado (to tow a crippled dodge home and eventually replace
my '84 Ranger). I ran it around town (6-7000 ft), mainly on the front tank, but a little on the back while I changed the oil and generally checked it out prior to the trip. No worries. As I climbed above 8500 ft, (using the full rear tank, front was about half full), I got a strong gas smell and it started to buck and jump. I pulled over, and opened both gas caps. The front hissed for a second, and was fine, the back vented (vapor only) for what seemed like a long time (minute, more? Seemed longer but I didn't time it). I put both caps on, and proceeded over the pass using the front tank, w/ no problems.
In the next town, I filled both tanks and put both caps on tight. Tried the rear tank, ran fine until I got up to altitude (above 8K, forget exactly), and repeated symptoms. I switched to the front tank, symptoms went away. I stopped and removed the cap from the rear, vapor flowed (but not for as long). I then put the back cap on loose (one full turn out from tight). I switched back to the rear (once I was moving again) and it worked fine for several more ups and downs.
I filled up again in Gunnison, and left the rear cap loose. It worked fine to well above 10,000 ft, then started bucking again. I switched to the front tank,and climbed the pass. The smell of fuel persisted, and at the top (first chance I took to look at it), there was liquid fuel spilling out of the (loose) cap I took the cap off and fuel gushed out for a long time (again, I didn't time it, it seemed long).
I replaced the rear cap, (with a $7 stant, not a motorcraft), and tightening the cap caused a similar problem on a lower pass later on. I can suck air in through all three caps, but not blow out through them w/ lung power. Running it loose was fine, and running it tight is fine if I don't climb. (All of these symptoms were while towing a minivan, if that makes any difference).
There seemed to be recent evidence of fuel being pushed out a connection up by the (square plastic) box that I assume is the unneeded fuel being sent back to the tank (it's fuel injected, 302). There seems to be one return line, which T's down by the front tank (how does it know which tank to send the fuel back to?).
The truck is now at 1600 feet, and runs fine with caps tight. I'm probably 10 hours away from anywhere I can test any possible fixes, but I'd like to know what the problem is/was.
I'm pretty sure I'm going to keep this truck, so will be buying a manual shortly but any help in the meantime is appreciated.
Thanks,
Bob
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I would defintetly check the vent.

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That's kind of the question. I don't have a shop manual yet, so am unsure how exactly it is supposed to be vented. I see two lines going to each tank, but (my understanding is) fuel injection requires two lines, the pump in the tank sends fuel forward, and the engine sends back what it doesn't need. Is there a third vent line on the top that I'm not seeing, or is it vented through the cap, or what? One (friend of a friend) story is that on the 1990 the tank wasn't vented -- he claimed he couldn't really fill his up due to this.
Any specific ideas for this model? (I will look around again for a vent this weekend).
Bob
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sounds like you have a switching valve problem. I would just replace it as long as it looks like nobody has messed with the hoses ect.
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BIG JOE :) opined in

yeah, bob... that'll make you forget your gas problems
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