Originally Posted by Sodar
Another gasket would help, but typically, the mating surfaces are not true and become warped. The powerboat repair shop I used to work for would always true them up on the disc sander before reinstalling them.
Pull the risers and check the mating surfaces with a straight edge... true up the one thats not square. A flat surface and some emery cloth works well. New gaskets and reinstall... shouldnt need any extra sealant.