There are two kinds of leaks, those that are caused by a brush error, i.e. a crack in the wall, and those caused by an entity outside of the map. Make sure none of your entities are outside of your map, and make sure no doors open so that they slide outside of the map.
If it is caused by a brush error:
Finding a leak, Bratty's guide:
way 1: Make a giant skybox around your map. A skybox is a big box of the SKY texture, hollowed out using the hollow tool. Make a bunch of light_environment entities outside of your map, but inside of the skybox, and make it a green icky color. Compile, and then walk around. If you see the green icky color anywhere, your leak way be in that area.
way 2: Make giant solid blocks on a section of your map. This should take up about %25 of your map. Compile. If there is still a leak, make the box bigger. Keep doing this til you have pin-pointed the leak out.
way 3: Start over! Once you have learned the fundementals of mapping, and know pretty much everything, don't try to debug your old ones, start over and make a great map to the best of your abilities.