Once a basement has been built and backfilled, it is harder and more expensive to keep it form leaking water. Concrete block foundations are by far the worst offenders and waterproofing them usually costs many hundreds of dollars. But by using inexpensive bentonite expanding clay and some common sense in where and how to place it, sealing up a leaky site-cast concrete basement is relatively simple. One method for eliminating seepage involves digging down alongside the crack on the outside and pouring in bentonite to seal it. This, plus an extension on the downspout, will take care of the leakage. The owner should be instructed not to disturb the bentonite fill when it gets wet and swells up. Although it may swell as much as a foot, he should leave it in place for two to three notches. After that he can remove the excess bentonite down to within an inch of the crack and fill in with black dirt. Bentonite will not harm plants or animals. Foundation cracks in locations where it is impossible to get access to the outside of a wall. Such as those beneath a thick stoop, have to be handled differently. The only sure way to stop such a leak form the inside is with a bleeder system. The reason it is called a bleeder is because an opening is left in the chipped out crack. To build a bleeder an opening is first cut through the basement floor. The opening should be 4 inches wide and centered on the crack. After cleaning and dampening the crack, a short length of smooth rope, similar to a clothesline rope but heavier, is placed into the point of the V. Next hydraulic cement mortar is packed tightly into the crack over the rope. While there is still enough rope not covered by mortar to permit pulling it, cementing is stopped and the mortar is allowed to set up. Filing and pulling is continued until the crack is fill. The crack is left open for drainage.