fuel pump loses it's prime, gas runs back to tank.

[m00n1957]

75 gmc 3/4 ton, dual tanks, 350, 1406 edelbrock carb. only way to get gas to fuel pump is pressurize the tank, runs good till shut off and gas drains back, have clear fuel filter and soon as shut off it empties. i've change fuel pump and tank selector. i've blown thru all lines, all are clear. doesn't matter if gas cap is on or off. no leaks. thanks in advance.

[El Dorado Jim]

I think the mechanical fuel pumps have a check valve, maybe it is bad

[Dead Parrot]

Does yours have a return line?
Does it work the same regardless of which tank is selected?
Does it start ok or take a lot of cranking to start?

I have noticed that those clear fuel filters often seem empty even if the engine is running and everything working fine.

[Tash4]

new fuel pump fixed this problem on my dads 85. truck had set for a year or so, still started and ran but would loose prime after a day.

[m00n1957]

Originally Posted by Dead Parrot

Does yours have a return line?
Does it work the same regardless of which tank is selected?
Does it start ok or take a lot of cranking to start?

I have noticed that those clear fuel filters often seem empty even if the engine is running and everything working fine.

doesn't matter what tank is selected, changed fuel pump again, still won't hold it's prime.

[cadillac_al]

It must be a pin hole in one of your lines. I found a pin hole on my K20 once by using a Mityvac to pull gas from the fuel tank. I could hear it hissing from under the hood. They may not all be that loud but you should be able to hear it. I just fixed a pinhole in my Caddy and the car starts so much easier now. It was a bear to start for the last few years (same with the K20). I found that leak once it started dripping.

[Alex V.]

What cadillac al said was what came to my mind first.

[m00n1957]

Originally Posted by cadillac_al

It must be a pin hole in one of your lines. I found a pin hole on my K20 once by using a Mityvac to pull gas from the fuel tank. I could hear it hissing from under the hood. They may not all be that loud but you should be able to hear it. I just fixed a pinhole in my Caddy and the car starts so much easier now. It was a bear to start for the last few years (same with the K20). I found that leak once it started dripping.

driver-side tank is full of gas, when i pressurize tank to get gas up to fuel pump i haven't found any gas leaking and i've put enough pressure that it will blow back on you if you release towel from gas inlet to quickly.

[Dead Parrot]

Is your problem just the issue of fuel not remaining visible in the filter when the motor is off? Does the truck start quickly or need a bunch of cranking to start?

On my 75 with dual tanks, I normally run one tank completely empty to the point the engines stumbles then switch to the other side. This means the pump and line from tank valve to the pump is empty of gas yet within a second or so after switching to the full tank, everything is running normally. This behavior seems to indicate that the fuel pump doesn't need a 'prime' to suck gas from a tank with gas in it.