when it's a lleak the rad file doesn't know how to calculate the light ammount in a map so it lights everything or it will be as dark as in your ass
Sort of.
The compile process Half-Life uses (from what I know at least) essentially bounces light around the entire room (provided something is lighting it) and then computes shadows based on that data. If there's a leak, then the light would theoretically bounce out the hole into the void and make RAD run infinitely as it's bouncing around out in the void. Talk about a long compile time... and the rad file? Do you mean the RAD exe?
The rad file is for defining texlights...