As above, but more detail if you need:
pop the pump off the block (three bolts iirc);
on the face of the pump that was against the block you will see a small metal disc about 3/8" to 1/2" dia with a small hole in the center, pressed in to a recess in the pump face, the spring is under there;
DON'T STARE RIGHT AT THE DISC, IT'S SPRING LOADED!
pull the disc out slowly, ready to catch the spring;
put a shim on top of the spring, push down HARD, reinstall disc into recess;
... and if all goes as usual, hunt around for the spring, disc, and shim on the floor somewhere and keep trying :-)