A leak means that there is a hole into the world outside your map.
Imagine the game as being in a vacuum, and the playable part is full of air. if there is nothing to separate the two you get a leak and that is what causes the effect with the rendering too as it cant define what else it needs to draw in place of the void.
To fix it you need to make sure that there is no gaps from your main level to the outside. You should use a brush with the tools/toolsskybox texture on it to make sky, and most other textures can be used for walls, ground etc.
If you are still getting a leak, you can go map>load pointfile and it will draw a line from an entity through the hole to the void. If the line goes through a brush remember that displacements, props and entitys (ie func_detail) DO NOT seal the world.