Accelerator pump issues after installing rebuild kit

Is it the pump or check valve? Remove the nozzle housing and add fluid to see if the ball seals the passage. If the fluid does not leak down, look elsewhere. If the check valve is bad, you can take an extra ball and install it, then take a hammer and punch and carefully tap it to get a good seal. Throw that ball away and install a new ball. I usually take a small screwdriver and "spread" the leather out before oiling it. Also polish the pump bore with WD-40 and some 600 wet or dry sand paper.