The secret to effective crack injection, whether using epoxy resins or polyurethane foams, is gradual introduction of the liquid polymer into the crack at low pressures (20 to 40 psi). This method requires some patience, but it allows the applicator to monitor the injection process and ensure that the crack is completely filled. Incomplete injection of a crack is the most common reason for crack repair failure.

To fill a typical crack in a residential foundation wall, injecting at pressures above 40 psi may not be effective. Liquids prefer to take the path of least resistance. At higher pressures, the liquid has enough force to and rise up the crack without filling the backside of the crack which is often narrower than the front of the crack. The applicator can only determine how the crack is being filled by watching the progress of the liquid traveling from port to port.

At pressures under 40 psi, the liquid epoxy or urethane can only get to the port above it by first filling to the back of the crack and then rise as it fills the crack. As a consequence, when the applicators see material coming out of the respective port above or adjacent to the port being filled, he (or she) knows that the crack is filled with product up to that point. At pressures greater than 40 psi, the liquid can overcome gravity and reach the port adjacent to the one being filled without filling small sections of the crack.