Re: Fuel tank pressure/suction
I think its a good approach in changing the pump.
I'll add to change all the hoses in the line as well; they do age and you can't see the inside. As for the steel lines, as long as they aren't leaking and not rusted, don't fix them.
As for the gas cap, you should be able to suck through the middle but not out. Another place for leakage is the outside seal to the filler tube. Is the filler tube smooth and no imperfections in the outer cap sealing area? The internal thread of the tube may be worn to not 'hold' the cap tight enough, the seal of the caps may not be thick enough or large enough diameter, or the backing may have lost its tension. Stant caps work well. After a hard run, immediately check the cap - if moist about the middle of the cap vent valve is leaking, if moist about the circumference of the cap, tube seal is leaking.
Incorporating the return line, the filter has a pressure regulating valve within, kindda. You might try a NAPA pump for a '69 350 as the mech pump supplies volume more than pressure. Pressure is resistance to volume - place thumb over the garden hose coupling and feel the hose expand, same principle.
I offer this for thought, if you are running faster than last year, mech pumps don't suck very well. As you accelerate, the fluid iin the line goes to the rear of the car - momentum thing. The pump suction must overcome this rearward phenomenal, if it can't, then starvation exists. Pumps deliver better than they can suck.
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