I have a 1988 F150 the fuel tanks are pressurizing and sending fuel back up the filler hoses I'm hoping somebody could help figure it out. Right now the rear leaf Springs are broken causing the of the rear of the bed is lower than the front of the truck. Is that what it could be?

 

Do you mean that both fuel caps have fuel running out when you remove them? Does it happen with the truck shut off, or while running? Does it only happen when trying to fill the tanks?
On my '88, if the filler necks are facing down hill, neither one will take fuel.
I did have the front tank fill the back tank due to debris in the Dual Function Reservoir (aka fuel tank selector valve) and a failing in tank fuel pump, in the front tank.

 

it does it both when running and shut off and it doesn't run it shoot out of the gas caps I can fuel tanks withoutany issue

 

Just so I understand, how do you fuel the tanks if fuel is shooting out?
The possibilities I see are the fuel filler necks leaning down hill; the classic cross fill issue, caused by the Dual Function Reservoir not isolating one of the tanks; a vent problem at the filler neck; or a vent problem at the EVAP system, which consists of a hose running from the top of the tanks to a charcoal canister in the engine compartment.

 

it doesn't always shoot out it is only when the truck has ran for a little bit which is why im with you thinking its the evap or the dual function reservoir. Because I can fuel the truck just fine

 

Does the truck run ok on both tanks? Do your fuel gauges work?

 

Runs a little wonky on the rear tank, but the front tank runs perfectly fine. And yes the fuel gauge works

 

By watching the fuel gauges, does the rear tank fill the front tank? Can you run the rear tank almost to empty?
I would run the rear tank down, or siphon it out, then lower it using two ratchet straps, and inspect the EVAP line and the electrical connection. I would do all of this outside, for safety purposes.
Once it's lowered, you can check for voltage and ground to the pump.
If it checks out electrically, raise it back up, put five gallons of fuel in it and test the output of the pump at the Dual Function Reservoir.
Out of the four connections at the rear of the Dual Function Reservoir, the pressure line from the rear pump is the bottom one, closest to the center of the truck.
Have a piece of fuel hose ready that's big enough to slide over the fitting on the fuel line. Disconnect the electrical connections at the high pressure fuel pump on the frame. Jumper pins two and six at the diagnostic plug under the hood by the driver's side fender. Disconnect the fitting from the Dual Function Reservoir, put the hose over it, and put it in a container.
With the rear tank selected, when you turn the key to the run position, the rear pump should pump into the container.
A good pump will pump 6 ounces of fuel in 5 seconds.
Every time I've disconnected my fuel lines at the Dual Function Reservoir, fuel has run out, without the pump being activated, so be ready for that. I wear safety glasses and a face shield.
You use the same process for testing the Dual Function Reservoir only you leave the high pressure pump connected, and disconnect the two return lines at the rear of the Dual Function Reservoir.
The return lines are the ones closest to the frame, top for front tank, bottom for rear tank. You use two pieces of hose, one over each return nipple on the Dual Function Reservoir.


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If the rear pump pumps 6 ounces in 5 seconds, the problem is most likely the Dual Function Reservoir. If it doesn't pump 6 ounces in 5 seconds and the electrical connections tested ok, then change the rear pump.
In that case, the Dual Function Reservoir might be ok and the rear pump is not making enough pressure to fully move the valves inside the DFR.