Preheat oven to 350°. Prepare loaf pan, line with parchment paper or grease.
Whisk the flour, baking powder, and salt together in large bowl.
In medium bowl, whisk the eggs. Once beaten, whisk in the buttermilk, brown sugar, oil, grapefruit juice, zest, vanilla extract.
Pour wet ingredients into dry ingredients and mix until combined. Do not overmix.
Spread batter into loaf pan. Bake for 45-50 minutes. Cake is done when toothpick inserted in the center comes out clean. If browning to quickly cover with foil.
Mix syrup ingredients until sugar is dissolved. Add more juice if necessary.
Remove cake and poke with holes. Pour syrup over cake and return to oven for 5 minutes.
Allow to cool.