Long Post-Fuel Issues - Too much Pressure?????

[71C10SWB]

Guys,
I am running out of answers.
I'll give you a rundown of the truck and recent upgrades:
71 C10 SWB, 350, TH350
Rebuilt 350 1 month ago.
New Fuel Pump
New Holley 600 w/adjustable Fuel Bowls (I'll explain later.)
New 3/8" fule line from tank to Carb
New Fuel Sender installed about the time trouble began.

Here's the deal: I've had the truck since March. It had a whipped 350/Powerglide combo in it. The engine had an Edelbrock 600 Carb on it and it leaked all over. The fuel sender didn't work and it was originally a 6-cylinder so it had 5/16" line from the tank to the carb.
Back in July, the Edelbrock carb started running worse than normal and got to the point whee it didn't want to run. I had a 750 Barry Grant Street Demon in the garage and slapped it on there to see if it was any better (I knew it was a little big, but wasn't too worried about it). It ran great for a day or two and then started leaking too (it had been sitting for quite a while so I wasn't surprised if the seals were all dried out.) I rebuilt an old Holley 600 that I had and put it on there. It ran good for a day or twom, but it didin't want to idle and the truck was gradually running worse.
I got sick of jacking with my old carbs and went down to the local Autozone (more due to convenience than the joy of shopping "The Zone". ). I bought a brand new Holley 600 with electric choke and thought my problems would be solved. Then I realized that it didin't have adjustable floats. It didin't take long before it was running like crap too. I knew the engine was really whipped and I figured I thought maybe the vacuum signal was so bad that it might be affecting the operation. since I couldn't keep it running, I decided to go for broke and do it right. Especially since I don't want to be stranded this winter.
I had my buddy's shop rebuild the engine. It has a mild cam (perfomer-like), new Performer RPM intake, new fuel pump, water pump, HEI distributor, etc... I also bought a new aluminum radiator and swapped in a TH350 that I had.
Everything ought to run ,ike new, right? Wrong!
The carb is giving me fits. I swapped in the rebuilt fuel bowls from my old 600 with adjustable floats becuase I thoguth maybe the float level wasn't right on the non-adjustable one's.
Here's what I have observed:
When I pull the fuel line to the carb after it's been running, it seems to have a lot of pressure built up in the fuel line. It actually spews out of the line when I start loosening the clamp.
Last night, I could see that I was getting a little bit of a leak around the sight plug on the rear bowl of the carb. I pulled it out to put on a new gasket and fuel came spewing out like it was under pressure.
I replaced the fuel line from the pump to the carb becuase there was an area that I thought might be questionable. When I pulled the line from the pump, there was fuel running out of the pump. I expected that to happen for a second or two, so I went and bent up a new line. Whan I came back, it was still trickling out. I've never seen that. It's almost like it's bypassing something in the pump.

I guess my main question is: Where is all of this pressure coming from and why????