1978 F100 300 six 2 wheel drive 3 speed. Is the fuel pump supposed to have some kind of a check valve in i?. If I park facing up a hill I need to prime the carb to start it. It's ok on level ground.

 

Ok think about this for a second or more ..........
The fuel line goes into the top of carbs and gas never up to where the needle is so there is no way in -_-_ fuel can run out of the carb back to the tank.
A hose or tube would need to go from the top of the carb to the float bowl bottom and there in none.

Now todays gas evaporates a lot faster than the gas did when your truck was made.
This is a problem with all older cars & trucks that sit for a bit, the gas in the bowl evaporates and now you have to crank the motor more to refill the bowl.
The other thing that does not help is the heat under the carbs that helps the fuel evaporate even faster.

On the 240 / 300 the intake & exh manifolds are bolted together and there is a flapper valve that sends hot exh. gases to the intake when the motor is cold but flops to the other side so no hot gases hit the intake.
This valve can become stuck sending hot gases to the intake and heating it up more than it needs to.
Now when you shut the motor off this hot manifold under the carb not heat the carb and the gas evaporates even faster.

Some times this can also cause a hot start issue. You drive for a little bit shut down to run in the store for a six pack and come up and need to crank for a long time again.
The gas in the carb has boiled coming out the bowl vent and may have flooded the motor.

Now IIRC the only thing on the bottom of the six popper carb is the inlet to the accel pump. Then again gas would need to go up and out the squirter and down into the carb & motor flooding it.
If the fuel line did drain back to the tank and there should still be fuel in the carb bowl a pump or 2 of the throttle and the motor should start and run long enough to pull fuel from the tank and keep running.

So first step is to make sure the valve in the exh manifold is open when the motor is up to temp or if stuck it is stuck open and not letting hot gases on the intake.
Don't know if it can be done on a six but on v8's you can get a non-metal carb spacer to keep heat from getting to the carb. Sometimes a thick gasket or 2 will work.
Another trick is to add 1/2 to 1 gallon of diesel fuel to a tank of gas at fill up and it will keep the gas from vaporizing from the heat but not hurt how the motor runs.

BTW I have a 300 six in my 81 f100 but I am running EFI exh. manifolds so no heat from the exh gases.
I do heat it with water after it leaves the heater core and I have not had any issues so far but the truck has only been on the road since November.
I do drive it to work just about everyday since then, 30+ miles 1 way. I run down the street to pick up food (2 miles each way) and to a buddies maybe 10 miles round trip so it is driven.
Dave ----