Hi - new to the forum, hoping someone can point me in the right direction

I have a 95 silverado 2500, 350 a/t

I have had no problems, for awhile, I had a friend put fuel in my truck, and he filled it right to the brim of the gascap, and as soon as i left to drive away it was sputtering and barely running. I limped home and the next day at the shop i syphoned some fuel out thinking it wasnt venting properly, after syphoning out like 25 litres, the truck ran great all day. After putting some fuel back in, i notice it runs, and only sputters and stalls when im gonig down a hill, or stop on a hill ( all the fuel goes to the front of the tank)


I do a yearly service of plugs,wires,cap,rotor,oil change, ff change, all that kinda stuff. I'm just pulling my hair out over this problem


I tried changing my fuel filter, driving without my charcoal canister, blowing out my fill tube and overflow tube

I can hear the fuel pump come on for a couple seconds when i turn my key on.

What would be my next step?

 

also another question, i cant find a diagram of lines off the tank

so im just assuming

1 to intake
1 return
and i think the last vent must go to the charcoal canister right?