If you discover a transmission fluid leak, you can stop it by using brake fluid as long as it is only leaking a few drops. If it a large leak, brake fluid will not be of any use. The general rule of thumb is to top off the transmission fluid using the brake fluid. Alternatively, you can use a stop leak product to thicken the transmission fluid and make it less prone to leaking. Once again, this should not be used on large leaks. For a large leak, take your car to a mechanic as soon as possible to prevent the vehicle from malfunctioning.