my sb would blow bubbles in the filter and do the same thing
I swaped a new pump on and fixed it for a while,then the bubbles would come back in 5-6 weeks of daily driving
swaped 5-6 pumps before I found it sucking rust into the pump and rupturing the diaphram
also found pin holes in the 68 steel line at the back by tank too
new line and one more new pump with filter before the pump fixed it
I removed the line and did not see any holes or wet spots.. but lots of rust/scale
slaped it down on the concret and there they are,I could see them after the rust got knocked of
check gas cap like said also