I have a 1995 12 valve. I replaced the lift pump with another stock unit, and when it's running, it works like it should.

If I leave it sit for a few days, the fuel seems to leak back down into the fuel tank, and then it takes several tries to reprime the system for the truck to run again.

Is there anything I can put inline to make sure it wont do this? I may have a leak, but it doesn't drip fuel. I'm sure some sort of 1 way valve would do the trick, but I've never heard of one.

 

You mean like a check valve?......Ben

 

On a truck that old I'd just replace the fuel lines with quality rubber hoses. Takes about 6-7ft of 10mm ID and 8mm ID and four hose clamps.
A check valve is a bad idea, since it impedes fuel flow on the suction side, which already is a weak point in the stock installation.

 

Not sure fuel delivery through the lines are the problem. It runs fine when primed. If there is a small air leak, I suspect it can leak back down into tank.

I'm trying to avoid purchasing one of those overpriced electric pump systems, but can't seem to figure out the current setup.

 

Imagine a pinhole just before the inlet of the lift pump.
If the engine is running, the suction of the lift pump will pull fuel and a minute amount of air, the injection pump has fuel and it runs.
If you stop there's no more suction, no more flow, and a little bit of air enters the hose, fuel goes back to the tank.
Next time trying to start you need to prime again.

The source of the problem is that the stock hoses get brittle and leaky over time. Replace the hoses and you are back to working fine. 6-8 ft of 10mm ID fuel hose, 6-8ft of 8mm ID fuel hose, 4 hose clamps. less than 1 hour of work. Less than 50$ in parts.
True problem fixed, not lipstick on a pig.