Have a ls1 with corvette filter regulator racetronix single pump,fuel pressure was 100psi running got a new filter now it’s 70running, put the gauge on my truck to compare it read 52 running so probably reads low if anything.
when I blew air through the return line air would go through but some air blow back after I let off,that normal? Should air flow really freely?

 

Take the line off at the fuel rail and put it in a gas can then turn the key on and see what kind of flow you've got. It definitely sounds like you've got a blockage of some kind